Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic earnings per common share:
Net income available to common shareholders	$ 28,095	$ 25,140	$ 77,105	$ 73,938
Weighted Average Number of Shares Outstanding, Basic	24,016	23,984	23,989	23,956
Basic earnings per common share (in dollars per share)	$ 1.17	$ 1.05	$ 3.21	$ 3.09
Diluted earnings per share:
Net income available to common shareholders	$ 28,095	$ 25,140	$ 77,105	$ 73,938
Weighted Average Number of Shares Outstanding, Basic	24,016	23,984	23,989	23,956
Effect of dilutive securities (in shares)	262	301	240	279
Weighted-average dilutive shares	24,278	24,285	24,229	24,235
Diluted earnings per common share (in dollars per share)	$ 1.16	$ 1.04	$ 3.18	$ 3.05