| Quarterly Financial Information [Table Text Block] |
| | | 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 | | | | | 2016 Quarter Ended | | | | (Dollars in thousands, except per share data) | | | | March 31 | | June 30 | | September 30 | | December 31 | | Interest income | | $ | 49,961 | | | $ | 52,050 | | | $ | 54,691 | | | $ | 56,200 | | | Interest expense | | | 5,782 | | | | 6,159 | | | | 6,773 | | | | 7,091 | | | Net interest income | | | 44,179 | | | | 45,891 | | | | 47,918 | | | | 49,109 | | | Provision for loan losses | | | 2,059 | | | | 3,800 | | | | 3,464 | | | | 4,075 | | | Net income available to common stockholders | | | 19,956 | | | | 18,853 | | | | 20,909 | | | | 21,714 | | | Net income per common share, basic | | $ | 0.38 | | | $ | 0.36 | | | $ | 0.40 | | | $ | 0.41 | | | Net income per common share, diluted | | $ | 0.37 | | | $ | 0.36 | | | $ | 0.39 | | | $ | 0.40 | | |