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Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 428,200 $ 378,600
Available-for-sale Securities, Gross Unrealized Loss 31,700 46,900
Investments [Abstract]    
Equity investments in unconsolidated subsidiaries 131,900 132,800
Miscellaneous investments 103,600 98,300
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 24,161 [1] 20,379 [2]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,555,999 [1] 1,528,993 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 272,437 [1] 260,877 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Emerging market debt funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 94,772 [1] 93,597 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Commodity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 106,571 [1] 106,571 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 137,176 [1] 132,190 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 125,410 [1] 128,630 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Other commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 151,048 [1] 151,048 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 27,369 [1] 32,764 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 127,841 [1] 104,913 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 25,345 [1] 21,751 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Municipal bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 5 [1] 13,609 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities [2]   2,785
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 275,101 [1] 270,779 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 188,763 [1] 189,100 [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 24,161 [1] 20,379 [2]
Alternative Investments, Fair Value Disclosure 833,351 [1],[3] 839,465 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,952,455 [1] 1,860,762 [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 Investments, 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 Investments, Fair Value Disclosure 98,876 [1],[3] 112,233 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 277,866 [1] 245,359 [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 Investments, Fair Value Disclosure 101,269 [1],[3] 97,543 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 101,269 [1] 97,543 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Commodity funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investments, Fair Value Disclosure 88,749 [1],[3] 92,091 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 88,749 [1] 92,091 [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 Investments, Fair Value Disclosure 194,912 [1],[3] 190,462 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 194,912 [1] 190,462 [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 Investments, Fair Value Disclosure 187,609 [1],[3] 187,647 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 187,609 [1] 187,647 [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 Investments, Fair Value Disclosure 161,936 [1],[3] 159,489 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 161,936 [1] 159,489 [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 Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 27,199 [1] 31,965 [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 Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 128,799 [1] 105,772 [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 Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 25,556 [1] 21,672 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 5 [1] 13,786 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investments, Fair Value Disclosure [2],[4]   0
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities [2]   2,816
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 Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 501,543 [1] 473,400 [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 Investments, Fair Value Disclosure 0 [1],[3] 0 [2],[4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 232,851 [1] 218,381 [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 24,161 [1] 20,379 [2]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 937,545 [1] 845,286 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 178,990 [1] 133,126 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Emerging market debt funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Commodity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Other commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Government securities    
Available-for-sale Securities [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    
Available-for-sale Securities [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    
Available-for-sale Securities [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 | Municipal bonds    
Available-for-sale Securities [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 | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 501,543 [1] 473,400 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 232,851 [1] 218,381 [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]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 181,559 [1] 176,011 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Emerging market debt funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Commodity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Other commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Government securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 27,199 [1] 31,965 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 128,799 [1] 105,772 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 25,556 [1] 21,672 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Municipal bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 5 [1] 13,786 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities [2]   2,816
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, 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]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Emerging market debt funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Commodity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Other commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Government securities    
Available-for-sale Securities [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    
Available-for-sale Securities [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    
Available-for-sale Securities [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 | Municipal bonds    
Available-for-sale Securities [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 | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities $ 0 [1] $ 0 [2]
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $131.9 million of equity investments in unconsolidated subsidiaries and $103.6 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 $132.8 million of equity investments in unconsolidated subsidiaries and $98.3 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.