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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, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale Securities, Gross Unrealized Gain $ 322,700 $ 328,800
Available-for-sale Securities, Gross Unrealized Loss 100,300 100,200
Investments [Abstract]    
Equity investments in unconsolidated subsidiaries 132,800 130,000
Miscellaneous investments 51,100 48,900
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 11,899 27,484
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,511,394 1,495,599
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 390,345 392,838
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | International equity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 264,340 259,114
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 108,882 105,965
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 73,577 61,816
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 24,320 24,444
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 76,952 73,061
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 18,117 13,726
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Municipal bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 47,088 49,255
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Asset-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 2,841 2,837
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 11,065 11,444
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Common stock    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 481,968 473,615
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 11,899 27,484
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 1,733,850 [1] 1,724,150 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 395,709 410,634
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | International equity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 242,312 231,122
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 158,915 157,528
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 100,576 84,750
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 23,213 21,356
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 70,723 65,276
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 17,343 12,801
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 49,902 51,589
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Asset-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 2,836 2,830
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 11,407 11,621
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 649,015 647,159
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 11,899 27,484
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 660,914 674,643
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 | Asset-backed 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 | Mortgage-backed 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 | Common stock    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities 649,015 647,159
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 175,424 165,473
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 0 0
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 0 0
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 23,213 21,356
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 70,723 65,276
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 17,343 12,801
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 49,902 51,589
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Asset-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 2,836 2,830
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 11,407 11,621
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 0 0
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 0 0
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 0
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 | Asset-backed 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 | Mortgage-backed 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 | 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 | Fair Value, Measurements, Net Asset Value (NAV) [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash Equivalents 0 [3] 0 [4]
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 897,512 [3] 884,034 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Commingled funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 395,709 [3] 410,634 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | International equity funds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 242,312 [3] 231,122 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Private equity investments    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 158,915 [3] 157,528 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Real estate    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities 100,576 [3] 84,750 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Government securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | U.S. corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | International corporate bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Municipal bonds    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Asset-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Mortgage-backed securities    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Debt Securities 0 [3] 0 [4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Fair Value, Measurements, Net Asset Value (NAV) [Member] | Common stock    
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Equity Securities $ 0 [3] $ 0 [4]
[1] 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 $51.1 million of miscellaneous investments.
[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $130.0 million of equity investments in unconsolidated subsidiaries and $48.9 million of miscellaneous investments.
[3] Based on the requirements of ASU 2015-07, investments measured at fair value using a NAV methodology have not been classified in the fair value hierarchy. See Note 2 for further information on the adoption of ASU 2015-07.
[4] Based on the requirements of ASU 2015-07, investments measured at fair value using a NAV methodology have not been classified in the fair value hierarchy. See Note 2 for further information on the adoption of ASU 2015-07.