Variable Interest Entities - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 1,759	$ 4,108	$ 90	$ 837
Proved natural gas and oil properties	62,941	58,594
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	56,196	39,043
Current liabilities	2,374	3,061
Non-Guarantor Subsidiaries [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	1	84	$ 90	$ 38
Current liabilities	13	68
Variable Interest Entities, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Cash and cash equivalents	1	1
Proved natural gas and oil properties	488	488
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	397	251
Current liabilities	$ 11	$ 15
Variable Interest Entities, Primary Beneficiary [Member] | Non-Guarantor Subsidiaries [Member] | Limited Partner [Member]
Variable Interest Entity [Line Items]
Percentage of acquisition	10.00%
Variable interest entity, carrying value of investment	$ 10
Variable Interest Entities, Primary Beneficiary [Member] | Non-Guarantor Subsidiaries [Member] | Limited Partner [Member] | Minimum [Member]
Variable Interest Entity [Line Items]
Royalty percentage minimum	7.00%
Royalty percentage maximum	7.00%
Variable Interest Entities, Primary Beneficiary [Member] | Non-Guarantor Subsidiaries [Member] | Limited Partner [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Other Commitment	$ 25
Royalty percentage minimum	22.50%
Royalty percentage maximum	22.50%
Majority-Owned Subsidiary, Unconsolidated [Member] | Non-Guarantor Subsidiaries [Member] | Limited Partner [Member]
Variable Interest Entity [Line Items]
Percentage of acquisition	90.00%
Majority-Owned Subsidiary, Unconsolidated [Member] | Non-Guarantor Subsidiaries [Member] | Limited Partner [Member] | Maximum [Member]
Variable Interest Entity [Line Items]
Other Commitment	$ 225