Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Instruments (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 20	$ 67	$ 3	$ 49
Total gains — realized/unrealized
Included in earnings	40	23	47	9
Included in OCI		(1)		(1)
Included in nuclear decommissioning obligation	3	(1)	7	(4)
Purchases	5	22	9	28
Transfers into Level 3	3	(20)	(5)	7
Transfers out of Level 3	(3)	(5)	7	(3)
Ending balance	68	85	68	85
Gains/(losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of June 30, 2017 and 2016	22	11	7	(7)
Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	18	17	17	17
Total gains — realized/unrealized
Included in earnings	0	0	1	0
Included in OCI		(1)		(1)
Included in nuclear decommissioning obligation	0	0	0	0
Purchases	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	18	16	18	16
Gains/(losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of June 30, 2017 and 2016	0	0	0	0
Trust Fund Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	58	52	54	54
Total gains — realized/unrealized
Included in earnings	0	0	0	0
Included in OCI		0		0
Included in nuclear decommissioning obligation	3	(1)	7	(4)
Purchases	0	0	0	1
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	61	51	61	51
Gains/(losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of June 30, 2017 and 2016	0	0	0	0
Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(56)	(2)	(68)	(22)
Total gains — realized/unrealized
Included in earnings	40	23	46	9
Included in OCI		0		0
Included in nuclear decommissioning obligation	0	0	0	0
Purchases	5	22	9	27
Transfers into Level 3	3	(20)	(5)	7
Transfers out of Level 3	(3)	(5)	7	(3)
Ending balance	(11)	18	(11)	18
Gains/(losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held as of June 30, 2017 and 2016	$ 22	$ 11	$ 7	$ (7)