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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 12,713	$ 22,107	$ 29,786	$ 37,702
Less: Net income allocated to unvested participating restricted stock	(428)	(356)	(1,000)	(635)
Net income available to common stockholders - basic	12,285	21,751	28,786	37,067
Effect of unvested participating restricted stock in two-class calculation	0	0	3	0
Net income available to common stockholders - diluted	$ 12,285	$ 21,751	$ 28,789	$ 37,067
Weighted average outstanding shares of common stock - basic (in shares)	17,544	17,719	17,623	17,707
Dilutive effect of stock options (in shares)	259	6	204	14
Weighted average outstanding shares of common stock - diluted (in shares)	17,803	17,725	17,827	17,721
Net income per common share - basic (USD per share)	$ 0.70	$ 1.23	$ 1.63	$ 2.09
Net income per common share - diluted (USD per share)	$ 0.69	$ 1.23	$ 1.61	$ 2.09