Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 155		$ 201
Accounts receivable, net	358		581
Other current assets	251		174
Total current assets	764		956
Intercompany receivables	0		0
Natural gas and oil properties, using the full cost method	24,823		24,180
Other	555		525
Less: Accumulated depreciation, depletion and amortization	(20,279)		(20,049)
Total property and equipment, net	5,099		4,656
Investments in subsidiaries (equity method)	0		0
Other long-term assets	682		185
TOTAL ASSETS	6,545		5,797	$ 7,042
Current liabilities:
Accounts payable	585		609
Other current liabilities	327		237
Total current liabilities	912		846
Intercompany payables	0		0
Long-term debt	2,267		2,318
Pension and other postretirement liabilities	39		46
Other long-term liabilities	245		225
Negative carrying amount of subsidiaries, net	0		0
Total long-term liabilities	2,551		2,589
Commitments and contingencies (Note 13)
Total equity	3,082	$ 2,937	2,362	$ 2,250	$ 2,193	$ 1,979
TOTAL LIABILITIES AND EQUITY	6,545		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	(7,894)		(7,932)
Natural gas and oil properties, using the full cost method	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	(7,917)		(7,956)
Current liabilities:
Accounts payable	0		0
Other current liabilities	0		0
Total current liabilities	0		0
Intercompany payables	(7,894)		(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	(2,441)		(3,199)
Total long-term liabilities	(2,441)		(3,199)
Commitments and contingencies (Note 13)
Total equity	2,418		3,175
TOTAL LIABILITIES AND EQUITY	(7,917)		(7,956)
Parent
Current assets:
Cash and cash equivalents	155		201
Accounts receivable, net	0		4
Other current assets	5		8
Total current assets	160		213
Intercompany receivables	7,894		7,932
Natural gas and oil properties, using the full cost method	0		0
Other	196		197
Less: Accumulated depreciation, depletion and amortization	(162)		(154)
Total property and equipment, net	34		43
Investments in subsidiaries (equity method)	0		0
Other long-term assets	45		19
TOTAL ASSETS	8,133		8,207
Current liabilities:
Accounts payable	72		113
Other current liabilities	193		115
Total current liabilities	265		228
Intercompany payables	0		0
Long-term debt	2,267		2,318
Pension and other postretirement liabilities	39		46
Other long-term liabilities	39		54
Negative carrying amount of subsidiaries, net	2,441		3,199
Total long-term liabilities	4,786		5,617
Commitments and contingencies (Note 13)
Total equity	3,082		2,362
TOTAL LIABILITIES AND EQUITY	8,133		8,207
Guarantors
Current assets:
Cash and cash equivalents	0		0
Accounts receivable, net	358		577
Other current assets	246		166
Total current assets	604		743
Intercompany receivables	0		0
Natural gas and oil properties, using the full cost method	24,769		24,128
Other	330		301
Less: Accumulated depreciation, depletion and amortization	(20,059)		(19,840)
Total property and equipment, net	5,040		4,589
Investments in subsidiaries (equity method)	23		24
Other long-term assets	637		166
TOTAL ASSETS	6,304		5,522
Current liabilities:
Accounts payable	513		496
Other current liabilities	134		122
Total current liabilities	647		618
Intercompany payables	7,892		7,932
Long-term debt	0		0
Pension and other postretirement liabilities	0		0
Other long-term liabilities	206		171
Negative carrying amount of subsidiaries, net	0		0
Total long-term liabilities	206		171
Commitments and contingencies (Note 13)
Total equity	(2,441)		(3,199)
TOTAL LIABILITIES AND EQUITY	6,304		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	54		52
Other	29		27
Less: Accumulated depreciation, depletion and amortization	(58)		(55)
Total property and equipment, net	25		24
Investments in subsidiaries (equity method)	0		0
Other long-term assets	0		0
TOTAL ASSETS	25		24
Current liabilities:
Accounts payable	0		0
Other current liabilities	0		0
Total current liabilities	0		0
Intercompany payables	2		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	$ 25		$ 24