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Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) (USD $)
 3 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2013
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Gross Unrealized Gain $ 258,600,000 $ 240,300,000
Available-for-sale Securities, Gross Unrealized Loss 45,800,000 58,500,000
Investments [Abstract]    
Equity investments in unconsolidated subsidiaries 86,300,000 87,100,000
Miscellaneous investments 43,400,000 41,900,000
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 15,854,000 33,281,000
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,450,576,000 1,445,197,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Commingled funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 476,011,000 457,986,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | International equity funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 78,812,000 78,812,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 60,912,000 52,143,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 49,224,000 45,564,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 34,176,000 34,304,000
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 78,362,000 80,275,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | International corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 15,223,000 15,025,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Municipal bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 261,106,000 241,112,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Common stock
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 380,896,000 406,695,000
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 15,854,000 33,281,000
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,663,395,000 [1] 1,627,026,000 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Commingled funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 483,409,000 452,227,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | International equity funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 82,710,000 81,671,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Private equity investments
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 73,801,000 62,696,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Real estate
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 62,954,000 57,368,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Government securities
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 28,822,000 27,628,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | U.S. corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 81,827,000 83,538,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | International corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 15,685,000 15,358,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Municipal bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 260,044,000 232,016,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Common stock
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 558,289,000 581,243,000
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 15,854,000 33,281,000
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 574,143,000 614,524,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Commingled funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | International equity funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 0
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 0
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 0
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 0
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 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | International corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 0
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 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Common stock
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 558,289,000 581,243,000
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 0
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 952,497,000 892,438,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Commingled funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 483,409,000 452,227,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | International equity funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 82,710,000 81,671,000
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 0
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 0
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 28,822,000 27,628,000
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 81,827,000 83,538,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | International corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 15,685,000 15,358,000
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 260,044,000 232,016,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Common stock
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 0 0
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 0
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 136,755,000 120,064,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Commingled funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | International equity funds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 0 0
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 73,801,000 62,696,000
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 62,954,000 57,368,000
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 0
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 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | International corporate bonds
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 0
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 0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Common stock
    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities $ 0 $ 0
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $86.3 million of equity investments in unconsolidated subsidiaries and $43.4 million of miscellaneous investments.
[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $87.1 million of equity investments in unconsolidated subsidiaries and $41.9 million of miscellaneous investments.