Condensed Consolidating Financial Information (Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 29			$ 201
Accounts receivable, net	323			581
Other current assets	281			174
Total current assets	633			956
Intercompany receivables	0			0
Natural gas and oil properties, using the full cost method	25,060			24,180
Other	522			525
Less: Accumulated depreciation, depletion and amortization	(20,383)			(20,049)
Total property and equipment, net	5,199			4,656
Investments in subsidiaries (equity method)	0			0
Other long-term assets	766			185
TOTAL ASSETS	6,598			5,797	$ 7,058
Current liabilities:
Current portion of long-term debt	52			0
Accounts payable	539			609
Other current liabilities	289			237
Total current liabilities	880			846
Intercompany payables	0			0
Long-term debt	2,219			2,318
Pension and other postretirement liabilities	39			46
Other long-term liabilities	325			225
Negative carrying amount of subsidiaries, net	0			0
Total long-term liabilities	2,583			2,589
Commitments and contingencies (Note 13)
Total equity	3,135	$ 3,082	$ 2,937	2,362	$ 2,205	$ 2,250	$ 2,193	$ 1,979
TOTAL LIABILITIES AND EQUITY	6,598			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	(7,957)			(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	(7,980)			(7,956)
Current liabilities:
Current portion of long-term debt	0
Accounts payable	0			0
Other current liabilities	0			0
Total current liabilities	0			0
Intercompany payables	(7,957)			(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	(2,382)			(3,199)
Total long-term liabilities	(2,382)			(3,199)
Commitments and contingencies (Note 13)
Total equity	2,359			3,175
TOTAL LIABILITIES AND EQUITY	(7,980)			(7,956)
Parent
Current assets:
Cash and cash equivalents	29			201
Accounts receivable, net	0			4
Other current assets	6			8
Total current assets	35			213
Intercompany receivables	7,957			7,932
Natural gas and oil properties, using the full cost method	0			0
Other	179			197
Less: Accumulated depreciation, depletion and amortization	(149)			(154)
Total property and equipment, net	30			43
Investments in subsidiaries (equity method)	0			0
Other long-term assets	99			19
TOTAL ASSETS	8,121			8,207
Current liabilities:
Current portion of long-term debt	52
Accounts payable	77			113
Other current liabilities	130			115
Total current liabilities	259			228
Intercompany payables	0			0
Long-term debt	2,219			2,318
Pension and other postretirement liabilities	39			46
Other long-term liabilities	87			54
Negative carrying amount of subsidiaries, net	2,382			3,199
Total long-term liabilities	4,727			5,617
Commitments and contingencies (Note 13)
Total equity	3,135			2,362
TOTAL LIABILITIES AND EQUITY	8,121			8,207
Guarantors
Current assets:
Cash and cash equivalents	0			0
Accounts receivable, net	323			577
Other current assets	275			166
Total current assets	598			743
Intercompany receivables	0			0
Natural gas and oil properties, using the full cost method	25,006			24,128
Other	314			301
Less: Accumulated depreciation, depletion and amortization	(20,176)			(19,840)
Total property and equipment, net	5,144			4,589
Investments in subsidiaries (equity method)	23			24
Other long-term assets	667			166
TOTAL ASSETS	6,432			5,522
Current liabilities:
Current portion of long-term debt	0
Accounts payable	462			496
Other current liabilities	159			122
Total current liabilities	621			618
Intercompany payables	7,955			7,932
Long-term debt	0			0
Pension and other postretirement liabilities	0			0
Other long-term liabilities	238			171
Negative carrying amount of subsidiaries, net	0			0
Total long-term liabilities	238			171
Commitments and contingencies (Note 13)
Total equity	(2,382)			(3,199)
TOTAL LIABILITIES AND EQUITY	6,432			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:
Current portion of long-term debt	0
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