Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	$ 37	$ 916	$ 1,111	$ 1,423
Accounts receivable, net	416	428
Other current assets	114	165
Total current assets	567	1,509
Natural gas and oil properties, using the full cost method	24,611	23,890
Gathering systems	1,322	1,315
Other	576	564
Less: Accumulated depreciation, depletion and amortization	(20,276)	(19,997)
Total property and equipment, net	6,233	5,772
Other long-term assets	242	240
TOTAL ASSETS	7,042	7,521	7,150
Accounts payable	642	533
Other current liabilities	216	247
Total current liabilities	858	780
Long-term debt	3,570	4,391
Pension and other postretirement liabilities	55	58
Other long-term liabilities	309	313
Total long-term liabilities	3,934	4,762
Commitments and contingencies
Total equity	2,250	1,979
TOTAL LIABILITIES AND EQUITY	7,042	7,521
Eliminations [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Intercompany receivables	(7,954)	(7,978)
Investments in subsidiaries (equity method)	(24)	(24)
TOTAL ASSETS	(7,978)	(8,002)
Intercompany payables	(7,954)	(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	(7,978)	(8,002)
Parent [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	29	914	1,105	1,416
Other current assets	8	10
Total current assets	37	924
Intercompany receivables	7,954	7,978
Other	209	207
Less: Accumulated depreciation, depletion and amortization	(149)	(134)
Total property and equipment, net	60	73
Other long-term assets	22	16
TOTAL ASSETS	8,073	8,991
Accounts payable	82	73
Other current liabilities	75	110
Total current liabilities	157	183
Long-term debt	3,570	4,391
Pension and other postretirement liabilities	55	58
Other long-term liabilities	11	13
Negative carrying amount of subsidiaries, net	2,030	2,367
Total long-term liabilities	5,666	6,829
Commitments and contingencies
Total equity	2,250	1,979
TOTAL LIABILITIES AND EQUITY	8,073	8,991
Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Cash and cash equivalents	8	2	$ 6	$ 7
Accounts receivable, net	416	428
Other current assets	106	155
Total current assets	530	585
Natural gas and oil properties, using the full cost method	24,557	23,834
Gathering systems	1,295	1,288
Other	367	357
Less: Accumulated depreciation, depletion and amortization	(20,070)	(19,804)
Total property and equipment, net	6,149	5,675
Investments in subsidiaries (equity method)	24	24
Other long-term assets	220	224
TOTAL ASSETS	6,923	6,508
Accounts payable	560	460
Other current liabilities	141	137
Total current liabilities	701	597
Intercompany payables	7,954	7,978
Other long-term liabilities	298	300
Total long-term liabilities	298	300
Commitments and contingencies
Total equity	(2,030)	(2,367)
TOTAL LIABILITIES AND EQUITY	6,923	6,508
Non-Guarantors [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Natural gas and oil properties, using the full cost method	54	56
Gathering systems	27	27
Less: Accumulated depreciation, depletion and amortization	(57)	(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