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Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Instruments (Details) - Level 3 - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance $ (51) $ 49
Total gains/(losses) realized/unrealized:    
Included in earnings 45 2
Included in nuclear decommissioning obligations 0 (1)
Purchases (23) (28)
Contracts reclassified to held-for-sale 4  
Transfers into Level 3 (1) (18)
Transfers out of Level 3 13 (55)
Ending balance (13) (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 at the end of period 8 (13)
Debt Securities    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 17 17
Total gains/(losses) realized/unrealized:    
Included in earnings 2 0
Included in nuclear decommissioning obligations 0 0
Purchases 0 0
Contracts reclassified to held-for-sale 0  
Transfers into Level 3 0 0
Transfers out of Level 3 0 0
Ending balance 19 17
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period 2 0
Trust Fund Investment    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance 0 54
Total gains/(losses) realized/unrealized:    
Included in earnings   0
Included in nuclear decommissioning obligations   (1)
Purchases   1
Transfers into Level 3   0
Transfers out of Level 3   (54)
Ending balance   0
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period   0
Derivative    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Beginning balance (68) (22)
Total gains/(losses) realized/unrealized:    
Included in earnings 43 2
Included in nuclear decommissioning obligations 0 0
Purchases (23) (29)
Contracts reclassified to held-for-sale 4  
Transfers into Level 3 (1) (18)
Transfers out of Level 3 13 (1)
Ending balance (32) (68)
Gains (losses) for the period included in earnings attributable to the change in unrealized gains or losses relating to assets or liabilities still held at the end of period $ 6 $ (13)