Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2015

Debt Disclosure [Abstract]

Schedule of Borrowing Base Utilization, Credit Facility [Table Text Block]

Interest and commitment fees are accrued based on the borrowing base utilization grid below.  Eurodollar loans accrue interest at the London Interbank Offered Rate plus the applicable margin from the utilization table below, and Alternate Base Rate (“ABR”) and swingline loans accrue interest at Prime plus the applicable margin from the utilization table below.  Commitment fees are accrued on the unused portion of the aggregate commitment amount and are included in interest expense in the accompanying statements of operations.

Borrowing Base Utilization Grid

Borrowing Base Utilization Percentage	<25%		≥25% <50%		≥50% <75%		≥75% <90%		≥90%
Eurodollar Loans	1.250	%	1.500	%	1.750	%	2.000	%	2.250	%
ABR Loans or Swingline Loans	0.250	%	0.500	%	0.750	%	1.000	%	1.250	%
Commitment Fee Rate	0.300	%	0.300	%	0.350	%	0.375	%	0.375	%

Schedule of Line of Credit Facilities [Table Text Block]

The following table presents the outstanding balance, total amount of letters of credit, and available borrowing capacity under the Credit Agreement as of February 17, 2016, December 31, 2015, and December 31, 2014:

	As of February 17, 2016		As of December 31, 2015		As of December 31, 2014
	(in thousands)
Credit facility balance (1)	$	243,000	$	202,000	$	166,000
Letters of credit (2)	$	200	$	200	$	808
Available borrowing capacity	$	1,256,800	$	1,297,800	$	1,333,192

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⁽¹⁾ Deferred financing costs attributable to the credit facility are presented as a component of other noncurrent assets on the accompanying balance sheets and thus are not deducted from the credit facility balance.

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

Schedule of Long-term Debt Instruments [Table Text Block]

The Senior Notes, net of unamortized deferred financing costs, line on the accompanying balance sheets as of December 31, 2015, and 2014, consisted of the following:

	As of December 31,
	2015						2014 (1)
	Senior Notes		Unamortized Deferred Financing Costs		Senior Notes, Net of Unamortized Deferred Financing Costs		Senior Notes		Unamortized Deferred Financing Costs		Senior Notes, Net of Unamortized Deferred Financing Costs
	(in thousands)
6.625% Notes due 2019	$	—	$	—	$	—	$	350,000	$	4,591	$	345,409
6.50% Notes due 2021	350,000		4,106		345,894		350,000		4,806		345,194
6.125% Notes due 2022	600,000		8,714		591,286		600,000		9,812		590,188
6.50% Notes due 2023	400,000		5,231		394,769		400,000		5,969		394,031
5.0% Notes due 2024	500,000		7,455		492,545		500,000		8,377		491,623
5.625% Notes due 2025	500,000		8,524		491,476		—		—		—
Total	$	2,350,000	$	34,030	$	2,315,970	$	2,200,000	$	33,555	$	2,166,445

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(1) Prior period amounts have been reclassified to conform to the current period presentation on the accompanying balance sheets. Please refer to the section Recently Issued Accounting Standards in Note 1 – Summary of Significant Accounting Policies for additional discussion.