Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per common share:
Net income	$ 87,554	$ 101,282	$ 99,473
Weighted Average Number of Shares Outstanding, Basic	29,686	23,998	23,966
Basic earnings per common share (in dollars per share)	$ 2.95	$ 4.22	$ 4.15
Diluted earnings per share:
Net income	$ 87,554	$ 101,282	$ 99,473
Weighted Average Number of Shares Outstanding, Basic	29,686	23,998	23,966
Effect of dilutive securities (in shares)	236	221	258
Weighted-average dilutive shares	29,922	24,219	24,224
Diluted earnings per common share (in dollars per share)	$ 2.93	$ 4.18	$ 4.11