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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net (loss) income	$ (451,718)	$ 24,273	$ (416,075)	$ 76,645
Less: Net loss (income) allocated to unvested participating restricted stock	8,496	(338)	6,921	(1,103)
Net (loss) income available to common stockholders - basic	(443,222)	23,935	(409,154)	75,542
Effect of unvested participating restricted stock in two-class calculation	0	1	5	3
Net (loss) income available to common stockholders - diluted	$ (443,222)	$ 23,936	$ (409,149)	$ 75,545
Weighted average outstanding shares of common stock - basic (in shares)	17,742	17,950	17,718	18,099
Dilutive effect of stock options (in shares)	0	91	0	102
Weighted average outstanding shares of common stock - diluted (in shares)	17,742	18,041	17,718	18,201
Net income per common share - basic (USD per share)	$ (24.98)	$ 1.33	$ (23.09)	$ 4.17
Net income per common share - diluted (USD per share)	$ (24.98)	$ 1.33	$ (23.09)	$ 4.15
Antidilutive securities excluded from computation of earnings per share (in shares)	1		11