Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 25,193	$ 20,162	$ 70,871	$ 63,364
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	44,415,213	45,150,104	44,505,544	45,223,988
Dilutive effect of stock-based compensation	390,708	428,874	379,368	481,636
Weighted average common shares outstanding for diluted earnings per common share computation	44,805,921	45,578,978	44,884,912	45,705,624
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.57	$ 0.45	$ 1.59	$ 1.40
Diluted earnings per common share	$ 0.56	$ 0.44	$ 1.58	$ 1.39
Performance Shares [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			218,556	165,489
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	135	5,884	95,886	5,884