Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 201				$ 916				$ 1,423				$ 15
Accounts receivable, net	581				428
Other current assets	174				165
Total current assets	956				1,509
Natural gas and oil properties, using the full cost method	24,180				23,890
Gathering systems	38				1,315
Other	487				564
Less: Accumulated depreciation, depletion and amortization	(20,049)				(19,997)
Total property and equipment, net	4,656				5,772
Other long-term assets	185				240
TOTAL ASSETS	5,797				7,521				7,076
Accounts payable	609				533
Other current liabilities	237				247
Total current liabilities	846				780
Long-term debt	2,318				4,391
Pension and other postretirement liabilities	46				58
Other long-term liabilities	225				313
Total long-term liabilities	2,589				4,762
Commitments and contingencies
Total equity	2,362	$ 2,205	$ 2,250	$ 2,193	1,979	$ 1,652	$ 1,573	$ 1,278	917	$ 1,123	$ 573	$ 1,148	2,282
TOTAL LIABILITIES AND EQUITY	5,797				7,521
Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Intercompany receivables	(7,932)				(7,978)
Investments in subsidiaries (equity method)	(24)				(24)
TOTAL ASSETS	(7,956)				(8,002)
Intercompany payables	(7,932)				(7,978)
Negative carrying amount of subsidiaries, net	(3,199)				(2,367)
Total long-term liabilities	(3,199)				(2,367)
Total equity	3,175				2,343
TOTAL LIABILITIES AND EQUITY	(7,956)				(8,002)
Parent [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	201				914				1,416				4
Accounts receivable, net	4
Other current assets	8				10
Total current assets	213				924
Intercompany receivables	7,932				7,978
Other	197				207
Less: Accumulated depreciation, depletion and amortization	(154)				(134)
Total property and equipment, net	43				73
Other long-term assets	19				16
TOTAL ASSETS	8,207				8,991
Accounts payable	113				73
Other current liabilities	115				110
Total current liabilities	228				183
Long-term debt	2,318				4,391
Pension and other postretirement liabilities	46				58
Other long-term liabilities	54				13
Negative carrying amount of subsidiaries, net	3,199				2,367
Total long-term liabilities	5,617				6,829
Total equity	2,362				1,979
TOTAL LIABILITIES AND EQUITY	8,207				8,991
Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents					2				$ 7				$ 11
Accounts receivable, net	577				428
Other current assets	166				155
Total current assets	743				585
Natural gas and oil properties, using the full cost method	24,128				23,834
Gathering systems	11				1,288
Other	290				357
Less: Accumulated depreciation, depletion and amortization	(19,840)				(19,804)
Total property and equipment, net	4,589				5,675
Investments in subsidiaries (equity method)	24				24
Other long-term assets	166				224
TOTAL ASSETS	5,522				6,508
Accounts payable	496				460
Other current liabilities	122				137
Total current liabilities	618				597
Intercompany payables	7,932				7,978
Other long-term liabilities	171				300
Total long-term liabilities	171				300
Total equity	(3,199)				(2,367)
TOTAL LIABILITIES AND EQUITY	5,522				6,508
Non-Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Natural gas and oil properties, using the full cost method	52				56
Gathering systems	27				27
Less: Accumulated depreciation, depletion and amortization	(55)				(59)
Total property and equipment, net	24				24
TOTAL ASSETS	24				24
Total equity	24				24
TOTAL LIABILITIES AND EQUITY	$ 24				$ 24