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
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 14,363	$ 25,543
Less: Net income allocated to unvested participating restricted stock	(264)	(382)
Net income available to common stockholders	14,099	25,161
Effect of unvested participating restricted stock in two-class calculation	0	1
Net income available to common stockholders - diluted	$ 14,099	$ 25,162
Weighted average outstanding shares of common stock - basic (in shares)	17,694	18,260
Dilutive effect of stock options (in shares)	43	113
Weighted average outstanding shares of common stock - diluted (in shares)	17,737	18,373
Net income per common share - basic (USD per share)	$ 0.80	$ 1.38
Net income per common share - diluted (USD per share)	$ 0.79	$ 1.37