| Note 21 - Fair Value Measurement (Tables)
 | 12 Months Ended | 
| Dec. 31, 2019 | 
|---|
| Notes Tables |  | 
| Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] | 
     
      
       |  |  | Fair Value Measurements at December 31, 2019 Using |  |  
       |  |  | Quoted Prices in |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       |  |  | Active Markets |  |  | Significant Other |  |  | Significant |  |  |  |  |  |  
       |  |  | for Identical |  |  | Observable Inputs |  |  | Unobservable |  |  |  |  |  |  
       |  |  | Assets (Level 1) |  |  | (Level 2) |  |  | Inputs (Level 3) |  |  | Total |  |  
       | Assets Measured on a Recurring Basis: |  | (In Thousands) |  |  
       | Available-for-sale securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | U.S. Treasury securities |  | $ | - |  |  | $ | 49,210 |  |  | $ | - |  |  | $ | 49,210 |  |  
       | Government agency securities |  |  |  |  |  |  | 18,386 |  |  |  |  |  |  |  | 18,386 |  |  
       | Mortgage-backed securities |  |  | - |  |  |  | 474,054 |  |  |  | - |  |  |  | 474,054 |  |  
       | State and municipal securities |  |  | - |  |  |  | 57,272 |  |  |  | - |  |  |  | 57,272 |  |  
       | Corporate debt |  |  | - |  |  |  | 153,881 |  |  |  | 6,596 |  |  |  | 160,477 |  |  
       | Total assets at fair value |  | $ | - |  |  | $ | 752,803 |  |  | $ | 6,596 |  |  | $ | 759,399 |  |  
     
      
       |  |  | Fair Value Measurements at December 31, 2018 Using |  |  
       |  |  | Quoted Prices in |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       |  |  | Active Markets |  |  | Significant Other |  |  | Significant |  |  |  |  |  |  
       |  |  | for Identical |  |  | Observable Inputs |  |  | Unobservable |  |  |  |  |  |  
       |  |  | Assets (Level 1) |  |  | (Level 2) |  |  | Inputs (Level 3) |  |  | Total |  |  
       | Assets Measured on a Recurring Basis: |  | (In Thousands) |  |  
       | Available-for-sale securities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | U.S. Treasury securities |  | $ | - |  |  | $ | 58,428 |  |  | $ | - |  |  | $ | 58,428 |  |  
       | Government agency securities |  |  |  |  |  |  | 18,565 |  |  |  |  |  |  |  | 18,565 |  |  
       | Mortgage-backed securities |  |  | - |  |  |  | 304,304 |  |  |  | - |  |  |  | 304,304 |  |  
       | State and municipal securities |  |  | - |  |  |  | 105,994 |  |  |  | - |  |  |  | 105,994 |  |  
       | Corporate debt |  |  | - |  |  |  | 96,375 |  |  |  | 6,518 |  |  |  | 102,893 |  |  
       | Total assets at fair value |  | $ | - |  |  | $ | 583,666 |  |  | $ | 6,518 |  |  | $ | 590,184 |  |  | 
| Fair Value Measurements, Nonrecurring [Table Text Block] | 
     
      
       |  |  | Fair Value Measurements at December 31, 2019 Using |  |  
       |  |  | Quoted Prices in |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       |  |  | Active Markets |  |  | Significant Other |  |  | Significant |  |  |  |  |  |  
       |  |  | for Identical |  |  | Observable |  |  | Unobservable |  |  |  |  |  |  
       |  |  | Assets (Level 1) |  |  | Inputs (Level 2) |  |  | Inputs (Level 3) |  |  | Total |  |  
       | Assets Measured on a Nonrecurring Basis: |  | (In Thousands) |  |  
       | Impaired loans |  | $ | - |  |  |  | - |  |  | $ | 33,344 |  |  | $ | 33,344 |  |  
       | Other real estate owned and repossessed assets |  |  | - |  |  |  | - |  |  |  | 8,178 |  |  |  | 8,178 |  |  
       | Total assets at fair value |  |  | - |  |  |  | - |  |  | $ | 41,522 |  |  | $ | 41,522 |  |  
     
      
       |  |  | Fair Value Measurements at December 31, 2018 Using |  |  
       |  |  | Quoted Prices in |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       |  |  | Active Markets |  |  | Significant Other |  |  | Significant |  |  |  |  |  |  
       |  |  | for Identical |  |  | Observable |  |  | Unobservable |  |  |  |  |  |  
       |  |  | Assets (Level 1) |  |  | Inputs (Level 2) |  |  | Inputs (Level 3) |  |  | Total |  |  
       | Assets Measured on a Nonrecurring Basis: |  | (In Thousands) |  |  
       | Impaired loans |  | $ | - |  |  | $ | - |  |  | $ | 30,463 |  |  | $ | 30,463 |  |  
       | Other real estate owned |  |  | - |  |  |  | - |  |  |  | 5,169 |  |  |  | 5,169 |  |  
       | Total assets at fair value |  | $ | - |  |  | $ | - |  |  | $ | 35,632 |  |  | $ | 35,632 |  |  | 
| Fair Value, by Balance Sheet Grouping [Table Text Block] | 
     
      
       |  |  | December 31, |  |  
       |  |  | 2019 |  |  | 2018 |  |  
       |  |  | Carrying Amount |  |  | Fair Value |  |  | Carrying Amount |  |  | Fair Value |  |  
       |  |  | (In Thousands) |  |  
       | Financial Assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Level 1 Inputs: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Cash and cash equivalents |  | $ | 530,127 |  |  | $ | 530,127 |  |  | $ | 458,050 |  |  | $ | 458,050 |  |  
       |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Level 2 Inputs: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Debt securities available for sale |  | $ | 752,803 |  |  | $ | 752,803 |  |  | $ | 583,666 |  |  | $ | 583,666 |  |  
       | Equity securities |  |  | - |  |  |  | - |  |  |  | 894 |  |  |  | 894 |  |  
       | Federal funds sold |  |  | 100,473 |  |  |  | 100,473 |  |  |  | 223,845 |  |  |  | 223,845 |  |  
       | Mortgage loans held for sale |  |  | 6,302 |  |  |  | 6,312 |  |  |  | 120 |  |  |  | 121 |  |  
       |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Level 3 Inputs: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Debt securities available for sale |  | $ | 6,596 |  |  | $ | 6,596 |  |  | $ | 6,518 |  |  | $ | 6,518 |  |  
       | Debt securities held to maturity |  |  | 250 |  |  |  | 250 |  |  |  | - |  |  |  | - |  |  
       | Loans, net |  |  | 7,184,867 |  |  |  | 7,132,542 |  |  |  | 6,464,899 |  |  |  | 6,398,604 |  |  
       |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Financial Liabilities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Level 2 Inputs: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
       | Deposits |  | $ | 7,530,433 |  |  | $ | 7,534,984 |  |  | $ | 6,915,708 |  |  | $ | 6,910,176 |  |  
       | Federal funds purchased |  |  | 470,749 |  |  |  | 470,749 |  |  |  | 288,725 |  |  |  | 288,725 |  |  
       | Other borrowings |  |  | 64,703 |  |  |  | 65,048 |  |  |  | 64,666 |  |  |  | 64,613 |  |  |