Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income	$ 95,636	$ 86,795	$ 84,401
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	44,511,774	45,184,091	44,615,060
Dilutive effect of stock-based compensation	398,622	462,327	595,501
Weighted average common shares outstanding for diluted earnings per common share computation	44,910,396	45,646,418	45,210,561
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.15	$ 1.92	$ 1.89
Diluted earnings per common share (in dollars per share)	$ 2.13	$ 1.90	$ 1.87
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	7,215	12,890
Stock Options
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)			5,000
Performance Shares [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	155,637	142,512	88,797