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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 21,280	$ 26,829	$ 35,643	$ 52,372
Less: Net income allocated to unvested participating restricted stock	(342)	(384)	(602)	(766)
Net income available to common stockholders - basic	20,938	26,445	35,041	51,606
Effect of unvested participating restricted stock in two-class calculation	0	1	0	1
Net income available to common stockholders - diluted	$ 20,938	$ 26,446	$ 35,041	$ 51,607
Weighted average outstanding shares of common stock - basic (in shares)	17,719	18,085	17,707	18,173
Dilutive effect of stock options (in shares)	6	103	14	107
Weighted average outstanding shares of common stock - diluted (in shares)	17,725	18,188	17,721	18,280
Net income per common share - basic (USD per share)	$ 1.18	$ 1.46	$ 1.98	$ 2.84
Net income per common share - diluted (USD per share)	$ 1.18	$ 1.45	$ 1.98	$ 2.82