| Note 7 - Deposits - Summary of Deposits (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017 | 
|---|---|---|
| Non-interest-bearing demand | $ 1,557,341 | $ 1,440,326 | 
| Interest-bearing checking | 4,624,909 | 4,029,147 | 
| Savings | 53,880 | 50,959 | 
| Time deposits, $250,000 and under | 257,925 | 228,540 | 
| Time deposits, over $250,000 | 421,653 | 342,702 | 
| Total deposits | $ 6,915,708 | $ 6,091,674 | 
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- Definition Amount of time deposit liabilities, including certificates of deposit, in denominations that are below the Federal Deposit Insurance Corporation (FDIC) insurance limit. No definition available. 
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- Definition The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs). Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of interest-bearing domestic checking deposit liabilities. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition The aggregate amount of all domestic and foreign noninterest-bearing deposits liabilities held by the entity. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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