| Quarterly Financial Information [Table Text Block] |
| | | 2018 Quarter Ended | | | | (Dollars in thousands, except per share data) | | | | March 31 | | June 30 | | September 30 | | December 31 | | Interest income | | $ | 74,009 | | | $ | 78,396 | | | $ | 84,058 | | | $ | 90,164 | | | Interest expense | | | 11,573 | | | | 13,874 | | | | 17,195 | | | | 21,306 | | | Net interest income | | | 62,436 | | | | 64,522 | | | | 66,863 | | | | 68,858 | | | Provision for loan losses | | | 4,139 | | | | 4,121 | | | | 6,624 | | | | 6,518 | | | Net income available to common stockholders | | | 32,603 | | | | 33,509 | | | | 34,560 | | | | 36,205 | | | Net income per common share, basic | | $ | 0.61 | | | $ | 0.63 | | | $ | 0.65 | | | $ | 0.68 | | | Net income per common share, diluted | | $ | 0.60 | | | $ | 0.62 | | | $ | 0.64 | | | $ | 0.67 | | | | | 2017 Quarter Ended | | | | (Dollars in thousands, except per share data) | | | | March 31 | | June 30 | | September 30 | | December 31 | | Interest income | | $ | 59,517 | | | $ | 63,538 | | | $ | 67,641 | | | $ | 72,060 | | | Interest expense | | | 7,465 | | | | 7,971 | | | | 9,245 | | | | 10,652 | | | Net interest income | | | 52,052 | | | | 55,567 | | | | 58,396 | | | | 61,408 | | | Provision for loan losses | | | 4,986 | | | | 4,381 | | | | 4,803 | | | | 9,055 | | | Net income available to common stockholders | | | 22,519 | | | | 24,133 | | | | 25,259 | | | | 21,119 | | | Net income per common share, basic | | $ | 0.42 | | | $ | 0.46 | | | $ | 0.48 | | | $ | 0.40 | | | Net income per common share, diluted | | $ | 0.41 | | | $ | 0.45 | | | $ | 0.47 | | | $ | 0.39 | | |