Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Basic earnings per common share:
Net income available to common shareholders	$ 24,494	$ 23,926
Weighted Average Number of Shares Outstanding, Basic	23,969	23,943
Basic earnings per common share (in dollars per share)	$ 1.02	$ 1.00
Diluted earnings per share:
Net income available to common shareholders	$ 24,494	$ 23,926
Weighted Average Number of Shares Outstanding, Basic	23,969	23,943
Effect of dilutive securities (in shares)	222	258
Weighted-average dilutive shares	24,191	24,201
Diluted earnings per common share (in dollars per share)	$ 1.01	$ 0.99