Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 22,222	$ 21,077	$ 43,202	$ 42,470
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	45,143,122	44,044,260	45,260,104	44,021,166
Dilutive effect of stock-based compensation	463,564	531,703	582,935	573,689
Weighted average common shares outstanding for diluted earnings per common share computation	45,606,686	44,575,963	45,843,039	44,594,855
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.49	$ 0.48	$ 0.95	$ 0.96
Diluted earnings per common share	$ 0.49	$ 0.47	$ 0.94	$ 0.95
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			166,362	107,651