Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Excise Taxes Collected	$ 46	$ 45	$ 45
Regulatory Liabilities	1,392	1,421
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	506
Other Investments	$ 255	26
Document Period End Date	Dec. 31, 2020
Days after the due date	45 days
Wholesale accounts receivable written off	$ 0	0	$ 0
Margin deposit	8	16
Letters of Credit Outstanding, Amount	$ 12	$ 15
Public Utilities, Allowance for Funds Used During Construction, Rate	6.90%	7.10%	7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest	$ 8	$ 5	$ 6
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity	$ 16	$ 10	$ 11
Depreciation expense rate	3.50%	3.60%	3.60%
Finite-Lived Intangible Assets, Accumulated Amortization	$ 388	$ 366
Amortization of Intangible Assets	64	$ 64	$ 59
Future Amortization Expense, Year One	57
Future Amortization Expense, Year Two	51
Future Amortization Expense, Year Three	42
Future Amortization Expense, Year Four	37
Future Amortization Expense, Year Five	25
Power Cost Deadband - Lower Threshold	15
Power Cost Deadband - Upper Threshold	$ 30
Public Utilities, Approved Return on Equity, Percentage	9.50%	9.50%
Quantifying Misstatement in Current Year Financial Statements, Amount	$ 4,136	$ 3,917
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation	3,864	4,095
Asset Retirement Obligation, Cash Paid to Settle	18	9	$ 5
Deferred Purchased Power Costs	114
Unbilled Receivables, Current	97	86
net variable power cost amount below the baseline	13
Trojan decommissioning [Member]
Property, Plant and Equipment [Line Items]
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	88	91
Asset Retirement Obligation Costs [Member]
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	37	54
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	0
Deferred Income Tax Charge [Member]
Property, Plant and Equipment [Line Items]
Regulatory Liabilities	239	260
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided	$ 0
AOCI Attributable to Parent [Member]
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects due to Tax Reform		$ (2)