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Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($)
$ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unrealized Gain on Securities $ 547 $ 560
Unrealized Loss on Securities 23 7
Investments [Abstract]    
Equity investments in unconsolidated subsidiaries 138 140
Miscellaneous investments 115 114
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 31 [1] 29 [2]
Investments [Abstract]    
Equity Securities 1,620 [1] 1,591 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 262 [1] 264 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 158 [1] 156 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments    
Investments [Abstract]    
Equity Securities 151 [1] 141 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate    
Investments [Abstract]    
Equity Securities 128 [1] 131 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   9
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 76 [1] 68 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 330 [1] 320 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 58 [1] 50 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. equities    
Investments [Abstract]    
Equity Securities 269 [1] 271 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 157 [1] 152 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 31 [1] 29 [2]
Alternative Investment, Fair Value Disclosure 665 [1],[3] 659 [2],[4]
Investments [Abstract]    
Equity Securities 2,144 [1] 2,143 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 90 [1],[3] 90 [2],[4]
Investments [Abstract]    
Equity Securities 289 [1] 307 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Emerging market debt funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 158 [1],[3] 166 [2],[4]
Investments [Abstract]    
Equity Securities 158 [1] 166 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 220 [1],[3] 198 [2],[4]
Investments [Abstract]    
Equity Securities 220 [1] 198 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Real estate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 197 [1],[3] 202 [2],[4]
Investments [Abstract]    
Equity Securities 197 [1] 202 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Other commingled funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure [2],[4]   3
Investments [Abstract]    
Equity Securities [2]   9
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Government securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 75 [1] 69 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | U.S. corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 323 [1] 322 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 56 [1] 50 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Equity Securities 568 [1] 557 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Equity Securities 227 [1] 234 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 31 [1] 29 [2]
Investments [Abstract]    
Equity Securities 1,025 [1] 1,043 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 199 [1] 217 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   6
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. equities    
Investments [Abstract]    
Equity Securities 568 [1] 557 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 227 [1] 234 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 0 [1] 0 [2]
Investments [Abstract]    
Equity Securities 454 [1] 441 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 75 [1] 69 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 323 [1] 322 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 56 [1] 50 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 0 [1] 0 [2]
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. corporate bonds    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Investments [Abstract]    
Equity Securities $ 0 [1] $ 0 [2]
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $138 million of equity investments in unconsolidated subsidiaries and $115 million of rabbi trust assets and miscellaneous investments.
[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $140 million of equity investments in unconsolidated subsidiaries and $114 million of rabbi trust assets and miscellaneous investments.
[3] Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
[4] Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.