Long-Term Debt (Details) $ in Thousands			3 Months Ended		9 Months Ended
	Feb. 23, 2015 USD ($)	Apr. 01, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Oct. 08, 2015 USD ($)	Apr. 01, 2015 USD ($)	Dec. 31, 2014 USD ($)	Nov. 06, 2014 USD ($)	Nov. 05, 2014 USD ($)	Dec. 31, 2009 agreement
Long-Term Debt
Total carrying amount			$ 737,487		$ 737,487				$ 500,000
Interest expense			16,722	$ 11,849	47,553	$ 34,659
Proceeds from Issuance of Senior Long-term Debt					236,475	$ 500,000
Number of credit agreements | agreement												2
Senior notes
Long-Term Debt
Total Principal Amount / Face Value			750,000		750,000				500,000
Less: unamortized discount			(12,513)		(12,513)
Total carrying amount			737,487		737,487				500,000
2022 Notes | Senior notes
Long-Term Debt
Total Principal Amount / Face Value		$ 500,000	500,000		500,000				$ 500,000
Stated interest rate (as a percent)		6.75%
Interest expense			8,400		25,300
Accrued interest			16,900		16,900
2023 Notes
Long-Term Debt
Interest expense			5,800		13,900
2023 Notes | Senior notes
Long-Term Debt
Total Principal Amount / Face Value	$ 250,000		250,000		250,000
Stated interest rate (as a percent)	9.25%
Interest expense					5,800
Discount of principal amount, as a percentage	94.59%
Proceeds from Issuance of Senior Long-term Debt	$ 236,500
Accrued interest			$ 1,000		$ 1,000
Revolver
Long-Term Debt
Repayment of borrowings		$ 308,000
Borrowing base							$ 510,000	$ 562,500		$ 625,000	$ 550,000
Average interest rates (as a percent)			2.31%	2.17%	2.40%	2.54%
Average outstanding balance			$ 100,000	$ 261,000	$ 156,700	$ 334,600
Total interest and commitment fees			$ 1,000	1,800	$ 4,000	6,700
Revolver | London Interbank Offered Rate (LIBOR) | Minimum
Long-Term Debt
Margin interest rate (as a percent)					1.50%
Revolver | London Interbank Offered Rate (LIBOR) | Maximum
Long-Term Debt
Margin interest rate (as a percent)					2.50%
Revolver | Federal Funds Effective Swap Rate
Long-Term Debt
Margin interest rate (as a percent)					0.50%
Revolver | Base rate | Minimum
Long-Term Debt
Margin interest rate (as a percent)					0.50%
Revolver | Base rate | Maximum
Long-Term Debt
Margin interest rate (as a percent)					1.50%
Revolver | One Month Adjust LIBO Rate
Long-Term Debt
Margin interest rate (as a percent)					1.00%
Term Loan
Long-Term Debt
Repayment of borrowings		$ 160,000
Total interest and commitment fees				0		3,600
Unamortized deferred financing costs				$ 3,800		$ 3,800