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Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unrealized Gain on Securities $ 543 $ 560
Unrealized Loss on Securities 18 7
Investments [Abstract]    
Equity investments in unconsolidated subsidiaries 141 140
Miscellaneous investments 117 114
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 41 [1] 29 [2]
Investments [Abstract]    
Equity Securities 1,621 1,591 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 270 [1] 264 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 157 [1] 156 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments    
Investments [Abstract]    
Equity Securities 142 [1] 141 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate    
Investments [Abstract]    
Equity Securities 118 [1] 131 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Other commingled funds    
Investments [Abstract]    
Equity Securities 4 [1] 9 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 78 [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 325 [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 55 [1] 50 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. equities    
Investments [Abstract]    
Equity Securities 278 [1] 271 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 153 [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 41 [1] 29 [2]
Alternative Investment, Fair Value Disclosure 641 [1],[3] 659 [2],[4]
Investments [Abstract]    
Equity Securities 2,146 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 316 [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 164 [1],[3] 166 [2],[4]
Investments [Abstract]    
Equity Securities 164 [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 198 [1],[3] 198 [2],[4]
Investments [Abstract]    
Equity Securities 198 [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 186 [1],[3] 202 [2],[4]
Investments [Abstract]    
Equity Securities 186 [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 3 [1],[3] 3 [2],[4]
Investments [Abstract]    
Equity Securities 4 [1] 9 [2]
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 77 [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 321 [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 53 [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 557 [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 229 [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 41 [1] 29 [2]
Investments [Abstract]    
Equity Securities 1,054 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 226 [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 1 [1] 6 [2]
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 557 [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 229 [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 451 441 [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 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Government securities    
Investments [Abstract]    
Available-for-sale Securities, Debt Securities 77 [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 321 [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 53 [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 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 0 [1] 0 [2]
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 $141 million of equity investments in unconsolidated subsidiaries and $117 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.