Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 9			$ 916	$ 989			$ 1,423
Accounts receivable, net	397			428
Other current assets	145			165
Current assets held for sale	64
Total current assets	615			1,509
Natural gas and oil properties, using the full cost method	24,880			23,890
Gathering systems	38			1,315
Other	479			564
Less: Accumulated depreciation, depletion and amortization	(19,928)			(19,997)
Total property and equipment, net	5,469			5,772
Other long-term assets	194			240
Long-term assets held for sale	780
TOTAL ASSETS	7,058			7,521	7,202
Accounts payable	563			533
Other current liabilities	213			247
Current liabilities held for sale	116
Total current liabilities	892			780
Long-term debt	3,572			4,391
Pension and other postretirement liabilities	50			58
Other long-term liabilities	162			313
Long-term liabilities held for sale	177
Total long-term liabilities	3,961			4,762
Commitments and contingencies
Total equity	2,205	$ 2,250	$ 2,193	1,979	1,652	$ 1,573	$ 1,278	917
TOTAL LIABILITIES AND EQUITY	7,058			7,521
Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Intercompany receivables	(7,966)			(7,978)
Investments in subsidiaries (equity method)	(24)			(24)
TOTAL ASSETS	(7,990)			(8,002)
Intercompany payables	(7,966)			(7,978)
Negative carrying amount of subsidiaries, net	(2,030)			(2,367)
Total long-term liabilities	(2,030)			(2,367)
Commitments and contingencies
Total equity	2,006			2,343
TOTAL LIABILITIES AND EQUITY	(7,990)			(8,002)
Parent Company [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	9			914	985			1,416
Other current assets	11			10
Total current assets	20			924
Intercompany receivables	7,966			7,978
Other	204			207
Less: Accumulated depreciation, depletion and amortization	(153)			(134)
Total property and equipment, net	51			73
Other long-term assets	21			16
TOTAL ASSETS	8,058			8,991
Accounts payable	122			73
Other current liabilities	69			110
Total current liabilities	191			183
Long-term debt	3,572			4,391
Pension and other postretirement liabilities	50			58
Other long-term liabilities	10			13
Negative carrying amount of subsidiaries, net	2,030			2,367
Total long-term liabilities	5,662			6,829
Commitments and contingencies
Total equity	2,205			1,979
TOTAL LIABILITIES AND EQUITY	8,058			8,991
Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents				2	$ 4			$ 7
Accounts receivable, net	397			428
Other current assets	134			155
Current assets held for sale	64
Total current assets	595			585
Natural gas and oil properties, using the full cost method	24,825			23,834
Gathering systems	11			1,288
Other	275			357
Less: Accumulated depreciation, depletion and amortization	(19,717)			(19,804)
Total property and equipment, net	5,394			5,675
Investments in subsidiaries (equity method)	24			24
Other long-term assets	173			224
Long-term assets held for sale	780
TOTAL ASSETS	6,966			6,508
Accounts payable	441			460
Other current liabilities	144			137
Current liabilities held for sale	116
Total current liabilities	701			597
Intercompany payables	7,966			7,978
Other long-term liabilities	152			300
Long-term liabilities held for sale	177
Total long-term liabilities	329			300
Commitments and contingencies
Total equity	(2,030)			(2,367)
TOTAL LIABILITIES AND EQUITY	6,966			6,508
Non-Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Natural gas and oil properties, using the full cost method	55			56
Gathering systems	27			27
Less: Accumulated depreciation, depletion and amortization	(58)			(59)
Total property and equipment, net	24			24
TOTAL ASSETS	24			24
Commitments and contingencies
Total equity	24			24
TOTAL LIABILITIES AND EQUITY	$ 24			$ 24