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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 26,829	$ 26,897	$ 52,372	$ 55,309
Less: Net income allocated to unvested participating restricted stock	(384)	(359)	(766)	(726)
Net income available to common stockholders	26,445	26,538	51,606	54,583
Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted	1	1	1	2
Net income available to common stockholders - diluted	$ 26,446	$ 26,539	$ 51,607	$ 54,585
Weighted average outstanding shares of common stock - basic (in shares)	18,085	18,763	18,173	18,819
Dilutive effect of stock options (in shares)	103	132	107	140
Weighted average outstanding shares of common stock - diluted (in shares)	18,188	18,895	18,280	18,959
Net income per common share - basic (USD per share)	$ 1.46	$ 1.41	$ 2.84	$ 2.90
Net income per common share -diluted (USD per share)	$ 1.45	$ 1.40	$ 2.82	$ 2.88