Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net (loss) income	$ 85,536	$ (451,718)	$ 21,280	$ 14,363	$ 21,347	$ 24,273	$ 26,829	$ 25,543	$ (330,539)	$ 97,992	$ 104,923
Less: net loss (income) allocated to unvested participating restricted stock									6,519	(1,387)	(1,400)
Net (loss) income available to common stockholders - basic									(324,020)	96,605	103,523
Effect of unvested participating restricted stock									0	2	4
Numerator - net (loss) income available to common shareholders - diluted									$ (324,020)	$ 96,607	$ 103,527
Weighted average outstanding shares of common stock - basic (shares)									17,725	18,030	18,637
Stock options (shares)									15	95	131
Weighted average outstanding shares of common stock - diluted (shares)									17,740	18,125	18,768
Basic (USD per share)	$ 4.68	$ (24.98)	$ 1.18	$ 0.80	$ 1.18	$ 1.33	$ 1.46	$ 1.38	$ (18.28)	$ 5.36	$ 5.55
Diluted (USD per share)	$ 4.67	$ (24.98)	$ 1.18	$ 0.79	$ 1.18	$ 1.33	$ 1.45	$ 1.37	$ (18.28)	$ 5.33	$ 5.52