Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income	$ 86,795	$ 84,401	$ 86,136
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	45,184,091	44,615,060	43,566,681
Dilutive effect of stock-based compensation	462,327	595,501	477,921
Weighted average common shares outstanding for diluted earnings per common share computation	45,646,418	45,210,561	44,044,602
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 1.92	$ 1.89	$ 1.98
Diluted earnings per common share (in dollars per share)	$ 1.90	$ 1.87	$ 1.96
Stock Options
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	12,890	5,000	21,372
Performance Shares [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	142,512	88,797	37,734