Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 20,162	$ 19,155	$ 63,364	$ 61,625
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	45,150,104	44,911,858	45,223,988	44,321,326
Dilutive effect of stock-based compensation	428,874	548,430	481,636	562,667
Weighted average common shares outstanding for diluted earnings per common share computation	45,578,978	45,460,288	45,705,624	44,883,993
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.45	$ 0.43	$ 1.40	$ 1.39
Diluted earnings per common share	$ 0.44	$ 0.42	$ 1.39	$ 1.37
Performance Shares [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			165,489	102,529
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			0	0
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			5,884