Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 366	$ 201
Accounts receivable, net	390	581
Other current assets	131	174
Total current assets	887	956
Intercompany receivables	0	0
Natural gas and oil properties, using the full cost method	24,486	24,180
Gathering systems	38	38
Other	509	487
Less: Accumulated depreciation, depletion and amortization	(20,162)	(20,049)
Total property and equipment, net	4,871	4,656
Investments in subsidiaries (equity method)	0	0
Other long-term assets	688	185
TOTAL ASSETS	6,446	5,797	$ 7,713
Current liabilities:
Accounts payable	626	609
Other current liabilities	317	237
Total current liabilities	943	846
Intercompany payables	0	0
Long-term debt	2,267	2,318
Pension and other postretirement liabilities	43	46
Other long-term liabilities	256	225
Negative carrying amount of subsidiaries, net	0	0
Total long-term liabilities	2,566	2,589
Commitments and contingencies (Note 13)
Total equity	2,937	2,362	$ 2,193	$ 1,979
TOTAL LIABILITIES AND EQUITY	6,446	5,797
Eliminations
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	0	0
Other current assets	0	0
Total current assets	0	0
Intercompany receivables	(7,744)	(7,932)
Natural gas and oil properties, using the full cost method	0	0
Gathering systems	0	0
Other	0	0
Less: Accumulated depreciation, depletion and amortization	0	0
Total property and equipment, net	0	0
Investments in subsidiaries (equity method)	(23)	(24)
Other long-term assets	0	0
TOTAL ASSETS	(7,767)	(7,956)
Current liabilities:
Accounts payable	0	0
Other current liabilities	0	0
Total current liabilities	0	0
Intercompany payables	(7,744)	(7,932)
Long-term debt	0	0
Pension and other postretirement liabilities	0	0
Other long-term liabilities	0	0
Negative carrying amount of subsidiaries, net	(2,594)	(3,199)
Total long-term liabilities	(2,594)	(3,199)
Commitments and contingencies (Note 13)
Total equity	2,571	3,175
TOTAL LIABILITIES AND EQUITY	(7,767)	(7,956)
Parent
Current assets:
Cash and cash equivalents	366	201
Accounts receivable, net	0	4
Other current assets	11	8
Total current assets	377	213
Intercompany receivables	7,744	7,932
Natural gas and oil properties, using the full cost method	0	0
Gathering systems	0	0
Other	197	197
Less: Accumulated depreciation, depletion and amortization	(159)	(154)
Total property and equipment, net	38	43
Investments in subsidiaries (equity method)	0	0
Other long-term assets	42	19
TOTAL ASSETS	8,201	8,207
Current liabilities:
Accounts payable	124	113
Other current liabilities	189	115
Total current liabilities	313	228
Intercompany payables	0	0
Long-term debt	2,267	2,318
Pension and other postretirement liabilities	43	46
Other long-term liabilities	47	54
Negative carrying amount of subsidiaries, net	2,594	3,199
Total long-term liabilities	4,951	5,617
Commitments and contingencies (Note 13)
Total equity	2,937	2,362
TOTAL LIABILITIES AND EQUITY	8,201	8,207
Guarantors
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	390	577
Other current assets	120	166
Total current assets	510	743
Intercompany receivables	0	0
Natural gas and oil properties, using the full cost method	24,433	24,128
Gathering systems	11	11
Other	312	290
Less: Accumulated depreciation, depletion and amortization	(19,947)	(19,840)
Total property and equipment, net	4,809	4,589
Investments in subsidiaries (equity method)	23	24
Other long-term assets	646	166
TOTAL ASSETS	5,988	5,522
Current liabilities:
Accounts payable	502	496
Other current liabilities	128	122
Total current liabilities	630	618
Intercompany payables	7,743	7,932
Long-term debt	0	0
Pension and other postretirement liabilities	0	0
Other long-term liabilities	209	171
Negative carrying amount of subsidiaries, net	0	0
Total long-term liabilities	209	171
Commitments and contingencies (Note 13)
Total equity	(2,594)	(3,199)
TOTAL LIABILITIES AND EQUITY	5,988	5,522
Non-Guarantors
Current assets:
Cash and cash equivalents	0	0
Accounts receivable, net	0	0
Other current assets	0	0
Total current assets	0	0
Intercompany receivables	0	0
Natural gas and oil properties, using the full cost method	53	52
Gathering systems	27	27
Other	0	0
Less: Accumulated depreciation, depletion and amortization	(56)	(55)
Total property and equipment, net	24	24
Investments in subsidiaries (equity method)	0	0
Other long-term assets	0	0
TOTAL ASSETS	24	24
Current liabilities:
Accounts payable	0	0
Other current liabilities	0	0
Total current liabilities	0	0
Intercompany payables	1	0
Long-term debt	0	0
Pension and other postretirement liabilities	0	0
Other long-term liabilities	0	0
Negative carrying amount of subsidiaries, net	0	0
Total long-term liabilities	0	0
Commitments and contingencies (Note 13)
Total equity	23	24
TOTAL LIABILITIES AND EQUITY	$ 24	$ 24