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		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income	$ 26,897	$ 19,167	$ 55,309	$ 39,991
Less: Net income allocated to unvested participating restricted stock	(359)	(307)	(726)	(649)
Net income available to common stockholders	26,538	18,860	54,583	39,342
Effect of unvested participating restricted stock in two-class calculation	1	1	2	2
Net income available to common stockholders - diluted	$ 26,539	$ 18,861	$ 54,585	$ 39,344
Weighted average outstanding shares of common stock - basic (in shares)	18,763	18,776	18,819	18,785
Dilutive effect of stock options (in shares)	132	179	140	218
Weighted average outstanding shares of common stock - diluted (in shares)	18,895	18,955	18,959	19,003
Net income per common share - basic (USD per share)	$ 1.41	$ 1.00	$ 2.90	$ 2.09
Net income per common share -diluted (USD per share)	$ 1.40	$ 1.00	$ 2.88	$ 2.07