Commitments (Long-Term Purchase Commitments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Electricity Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	$ 827	$ 747	$ 420
Purchase Obligation, Due in Next Twelve Months	786
Purchase Obligation, Due in Second Year	602
Purchase Obligation, Due in Third Year	371
Purchase Obligation, Due in Fourth Year	234
Purchase Obligation, Due in Fifth Year	393
Future Commitments Thereafter	5,187
Future Commitments Total	7,573
Fuel Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,838	1,619	1,790
Purchase Obligation, Due in Next Twelve Months	1,494
Purchase Obligation, Due in Second Year	1,027
Purchase Obligation, Due in Third Year	882
Purchase Obligation, Due in Fourth Year	575
Purchase Obligation, Due in Fifth Year	463
Future Commitments Thereafter	1,734
Future Commitments Total	6,175
Other Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,671	$ 1,945	$ 817
Purchase Obligation, Due in Next Twelve Months	1,375
Purchase Obligation, Due in Second Year	681
Purchase Obligation, Due in Third Year	336
Purchase Obligation, Due in Fourth Year	561
Purchase Obligation, Due in Fifth Year	213
Future Commitments Thereafter	778
Future Commitments Total	$ 3,944