Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic earnings per common share:
Net income available to common shareholders	$ 24,516	$ 24,872	$ 49,010	$ 48,798
Weighted Average Number of Shares Outstanding, Basic	23,995	23,981	23,977	23,947
Basic earnings per common share (in dollars per share)	$ 1.02	$ 1.04	$ 2.04	$ 2.04
Diluted earnings per share:
Net income available to common shareholders	$ 24,516	$ 24,872	$ 49,010	$ 48,798
Weighted Average Number of Shares Outstanding, Basic	23,995	23,981	23,977	23,947
Effect of dilutive securities (in shares)	242	277	228	267
Weighted-average dilutive shares	24,237	24,258	24,205	24,214
Diluted earnings per common share (in dollars per share)	$ 1.01	$ 1.03	$ 2.02	$ 2.02