Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 25,357	$ 24,257	$ 26,897	$ 28,412	$ (22,425)	$ 18,887	$ 19,167	$ 20,824	$ 104,923	$ 36,453	$ 72,037
Less: Net income allocated to unvested participating restricted stock									(1,400)	(521)	(1,200)
Net income available to common stockholders - basic									103,523	35,932	70,837
Effect of unvested participating restricted stock									4	0	4
Numerator - net income available to common shareholders - diluted									$ 103,527	$ 35,932	$ 70,841
Weighted average outstanding shares of common stock - basic									18,637	18,753	18,871
Stock options									131	203	270
Weighted average outstanding shares of common stock - diluted									18,768	18,956	19,141
Basic (USD per share)	$ 1.36	$ 1.29	$ 1.41	$ 1.49	$ (1.18)	$ 0.99	$ 1.00	$ 1.09	$ 5.55	$ 1.92	$ 3.75
Diluted (USD per share)		$ 1.28	$ 1.40	$ 1.47	$ 1.35	$ 0.99	$ 1.00	$ 1.08	$ 5.52	$ 1.90	$ 3.70