Commitments (Long-Term Purchase Commitments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Electricity Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	$ 756	$ 1,597	$ 827
Purchase Obligation, Due in Next Twelve Months	700
Purchase Obligation, Due in Second Year	500
Purchase Obligation, Due in Third Year	447
Purchase Obligation, Due in Fourth Year	434
Purchase Obligation, Due in Fifth Year	434
Future Commitments Thereafter	5,037
Future Commitments Total	7,552
Fuel Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,573	1,824	1,838
Purchase Obligation, Due in Next Twelve Months	1,370
Purchase Obligation, Due in Second Year	815
Purchase Obligation, Due in Third Year	609
Purchase Obligation, Due in Fourth Year	495
Purchase Obligation, Due in Fifth Year	457
Future Commitments Thereafter	1,445
Future Commitments Total	5,191
Other Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,506	$ 1,684	$ 1,671
Purchase Obligation, Due in Next Twelve Months	1,904
Purchase Obligation, Due in Second Year	636
Purchase Obligation, Due in Third Year	605
Purchase Obligation, Due in Fourth Year	570
Purchase Obligation, Due in Fifth Year	526
Future Commitments Thereafter	1,816
Future Commitments Total	$ 6,057