Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions		9 Months Ended	42 Months Ended	180 Months Ended
	Jun. 30, 2019	Sep. 30, 2020	Jul. 01, 2022	Dec. 31, 2032	May 31, 2019
South Texas Divestiture
Loss Contingencies [Line Items]
Deficiency fee liability		$ (321)
Receivable from buyer		71
Credit support from third parties		100
South Texas Divestiture | Maximum
Loss Contingencies [Line Items]
Retained obligation percentage	100.00%
Deficiency fee liability		460
South Texas Divestiture | Forecast
Loss Contingencies [Line Items]
Retained obligation percentage			100.00%
South Texas Divestiture | Forecast | Maximum
Loss Contingencies [Line Items]
Retained obligation percentage			100.00%
Buyer recovery percentage			18.00%
Raton transportation commitments
Loss Contingencies [Line Items]
Credit support from third parties		50
Deficiency Fees Paid and Reimbursed		9
Raton transportation commitments | Maximum
Loss Contingencies [Line Items]
Deficiency fee liability		80
Raton transportation commitments | Forecast | Maximum
Loss Contingencies [Line Items]
Retained obligation percentage				100.00%
South Texas Divestiture | South Texas Divestiture
Loss Contingencies [Line Items]
Receivable from buyer					$ 72
South Texas Divestiture | Other current liabilities | South Texas Divestiture
Loss Contingencies [Line Items]
Monetary sanctions		$ (131)