Commitments (Long-Term Purchase Commitments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Electricity Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	$ 747	$ 420	$ 545
Future Commitments 2018	581
Future Commitments 2019	508
Future Commitments 2020	440
Future Commitments 2021	469
Future Commitments 2022	438
Future Commitments Thereafter	2,065
Future Commitments Total	4,501
Fuel Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,619	1,790	1,262
Future Commitments 2018	1,759
Future Commitments 2019	1,051
Future Commitments 2020	591
Future Commitments 2021	538
Future Commitments 2022	454
Future Commitments Thereafter	1,466
Future Commitments Total	5,859
Other Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,945	$ 817	$ 1,833
Future Commitments 2018	1,488
Future Commitments 2019	931
Future Commitments 2020	470
Future Commitments 2021	234
Future Commitments 2022	547
Future Commitments Thereafter	1,314
Future Commitments Total	$ 4,984