| Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] |
| | | Three Months Ended June 30, | | Six Months Ended June 30, | | | | 2018 | | 2017 | | 2018 | | 2017 | | | | (In Thousands, Except Shares and Per Share Data) | | Earnings per common share | | | | | | | | | | | | | | | | | | Weighted average common shares outstanding | | | 53,150,142 | | | | 52,864,761 | | | | 53,116,420 | | | | 52,805,378 | | | Net income available to common stockholders | | $ | 33,509 | | | $ | 24,133 | | | $ | 66,112 | | | $ | 46,652 | | | Basic earnings per common share | | $ | 0.63 | | | $ | 0.46 | | | $ | 1.24 | | | $ | 0.88 | | | | | | | | | | | | | | | | | | | | | Weighted average common shares outstanding | | | 53,150,142 | | | | 52,864,761 | | | | 53,116,420 | | | | 52,805,378 | | | Dilutive effects of assumed conversions and exercise of stock options and warrants | | | 1,045,881 | | | | 1,235,843 | | | | 1,073,326 | | | | 1,311,694 | | | Weighted average common and dilutive potential common shares outstanding | | | 54,196,023 | | | | 54,100,604 | | | | 54,189,746 | | | | 54,117,072 | | | Net income available to common stockholders | | $ | 33,509 | | | $ | 24,133 | | | $ | 66,112 | | | $ | 46,652 | | | Diluted earnings per common share | | $ | 0.62 | | | $ | 0.45 | | | $ | 1.22 | | | $ | 0.86 | | |