Net Income per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 24,257	$ 18,887	$ 79,566	$ 58,878
Less: Net income allocated to unvested participating restricted stock	(316)	(279)	(1,042)	(927)
Net income available to common stockholders	23,941	18,608	78,524	57,951
Effect of unvested participating restricted stock in two-class calculation	1	1	3	3
Net income available to common stockholders - diluted	$ 23,942	$ 18,609	$ 78,527	$ 57,954
Weighted average outstanding shares of common stock - basic (in shares)	18,573	18,703	18,737	18,757
Dilutive effect of stock options (in shares)	133	187	137	207
Weighted average outstanding shares of common stock - diluted (in shares)	18,706	18,890	18,874	18,964
Net income per common share - basic (USD per share)	$ 1.29	$ 0.99	$ 4.19	$ 3.09
Net income per common share -diluted (USD per share)	$ 1.28	$ 0.99	$ 4.16	$ 3.06