Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted	$ 106,521	$ 95,636	$ 86,795
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	51,429,366	44,511,774	45,184,091
Dilutive effect of stock-based compensation (in shares)	473,843	398,622	462,327
Weighted average common shares outstanding for diluted earnings per common share computation	51,903,209	44,910,396	45,646,418
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.07	$ 2.15	$ 1.92
Diluted earnings per common share (in dollars per share)	$ 2.05	$ 2.13	$ 1.90
Restricted Stock
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	83,635	7,215	12,890
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	113,874	155,637	142,512