Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Borrowing Base Utilization Grid, Credit Facility [Table Text Block]

The borrowing base utilization grid under the Credit Agreement is as follows:

Borrowing Base Utilization Percentage	<25%		≥25% <50%		≥50% <75%		≥75% <90%		≥90%
Eurodollar Loans	1.750	%	2.000	%	2.250	%	2.500	%	2.750	%
ABR Loans or Swingline Loans	0.750	%	1.000	%	1.250	%	1.500	%	1.750	%
Commitment Fee Rate	0.300	%	0.300	%	0.350	%	0.375	%	0.375	%

Schedule of Credit Agreement Facilities [Table Text Block]

The following table presents the outstanding balance, total amount of letters of credit outstanding, and available borrowing capacity under the Credit Agreement as of February 14, 2018, December 31, 2017, and December 31, 2016:

	As of February 14, 2018		As of December 31, 2017		As of December 31, 2016
	(in thousands)
Credit facility balance (1)	$	—	$	—	$	—
Letters of credit (2)	200		200		200
Available borrowing capacity	924,800		924,800		1,164,800
Total aggregate lender commitment amount	$	925,000	$	925,000	$	1,165,000

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⁽¹⁾  Unamortized deferred financing costs attributable to the credit facility are presented as a component of other noncurrent assets on the accompanying balance sheets and totaled $3.1 million and $5.9 million as of December 31, 2017, and 2016, respectively.

⁽²⁾  Letters of credit outstanding reduce the amount available under the credit facility on a dollar-for-dollar basis.

Schedule of Senior Notes [Table Text Block]

Senior Notes, net of unamortized deferred financing costs line on the accompanying balance sheets as of December 31, 2017, and 2016, consisted of the following:

	As of December 31,
	2017						2016
	Principal Amount		Unamortized Deferred Financing Costs		Senior Notes, Net of Unamortized Deferred Financing Costs		Principal Amount		Unamortized Deferred Financing Costs		Senior Notes, Net of Unamortized Deferred Financing Costs
	(in thousands)
6.50% Senior Notes due 2021	$	344,611	$	2,656	$	341,955	$	346,955	$	3,372	$	343,583
6.125% Senior Notes due 2022	561,796		5,800		555,996		561,796		6,979		554,817
6.50% Senior Notes due 2023	394,985		3,707		391,278		394,985		4,436		390,549
5.0% Senior Notes due 2024	500,000		5,610		494,390		500,000		6,533		493,467
5.625% Senior Notes due 2025	500,000		6,714		493,286		500,000		7,619		492,381
6.75% Senior Notes due 2026	500,000		7,242		492,758		500,000		8,078		491,922
Total	$	2,801,392	$	31,729	$	2,769,663	$	2,803,736	$	37,017	$	2,766,719

Schedule of Senior Convertible Notes [Table Text Block]

The net carrying amount of the liability component of the Senior Convertible Notes, as reflected on the accompanying balance sheets, consisted of the following as of December 31, 2017 and 2016:

	As of December 31,
	2017			2016
	(in thousands)
Principal amount of Senior Convertible Notes	$	172,500		$	172,500
Unamortized debt discount	(30,183		)	(37,513		)
Unamortized deferred financing costs	(3,210		)	(4,131		)
Net carrying amount	$	139,107		$	130,856

The net carrying amount of the equity component of the Senior Convertible Notes recorded in additional paid-in capital on the accompanying balance sheets consisted of the following as of December 31, 2017 and 2016:

	As of December 31,
	2017			2016
	(in thousands)
Equity component due to allocation of proceeds to equity	$	40,217		$	40,217
Related issuance costs	(1,375		)	(1,375		)
Deferred tax liability	(5,267		)	(5,267		)
Net carrying amount	$	33,575		$	33,575