Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory Assets	$ 593	$ 500
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	506
Regulatory Liabilities	1,392	1,421
Removal Costs [Member]
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	0
Regulatory Liabilities	1,016	1,021
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	0
Regulatory Liabilities	239	260
Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	0
Regulatory Liabilities	$ 37	54
Revenue Subject to Refund [Member]
Regulatory Assets [Line Items]
Regulatory Current Asset, End Date for Recovery	2020
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 0
Regulatory Liabilities	0	23
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	36
Regulatory Liabilities	82	61
Regulatory Assets Earning a Rate of Return at the Approved Rate	46
RemainingAmountsOfRegulatoryLiabilitesForWhichNoInterstAccruesDuringRecoveryPeriodIsProvided	54
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets		2
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	18
Regulatory Liabilities	0
Earning a return [Member]
Regulatory Assets [Line Items]
Regulatory Liabilities	1,338
Other Regulatory Assets Earning a Return [Member]
Regulatory Assets [Line Items]
Regulatory Assets	87
Earning a return [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 87
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Current Asset, End Date for Recovery	2035
Regulatory Assets	$ 0	95
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	124
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0	213
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 240
Loss on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Current Asset, End Date for Recovery	2050
Regulatory Assets	$ 0	26
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 25
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Current Asset, End Date for Recovery	2059
Regulatory Assets	$ 0
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	95	94
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Regulatory Assets	109	$ 72
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 22