Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic earnings per common share:
Net income	$ 101,282	$ 99,473	$ 74,364
Weighted Average Number of Shares Outstanding, Basic	23,998,000	23,966,000	23,897,000
Basic earnings per common share (in dollars per share)	$ 4.22	$ 4.15	$ 3.11
Diluted earnings per share:
Net income	$ 101,282	$ 99,473	$ 74,364
Weighted Average Number of Shares Outstanding, Basic	23,998,000	23,966,000	23,897,000
Effect of dilutive securities (in shares)	221	258	257
Weighted-average dilutive shares	24,219,000	24,224,000	24,154,000
Diluted earnings per common share (in dollars per share)	$ 4.18	$ 4.11	$ 3.08