Commitments (Long-Term Purchase Commitments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Electricity Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	$ 2,447	$ 2,120	$ 2,475
Future Commitments 2017	2,513
Future Commitments 2018	2,507
Future Commitments 2019	2,367
Future Commitments 2020	2,704
Future Commitments 2021	2,750
Future Commitments Thereafter	20,265
Future Commitments Total	33,106
Fuel Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,790	1,262	1,521
Future Commitments 2017	1,609
Future Commitments 2018	724
Future Commitments 2019	489
Future Commitments 2020	451
Future Commitments 2021	465
Future Commitments Thereafter	1,425
Future Commitments Total	5,163
Other Purchase Contracts
Long-Term Purchase Commitment [Line Items]
Purchases Under Long Term Contracts	1,093	$ 2,110	$ 1,367
Future Commitments 2017	2,966
Future Commitments 2018	1,865
Future Commitments 2019	1,395
Future Commitments 2020	956
Future Commitments 2021	815
Future Commitments Thereafter	6,012
Future Commitments Total	$ 14,009