Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) (USD $)	Jun. 30, 2013		Dec. 31, 2012
Non-Derivative Instruments Fair Value Measurements [Abstract]
Nuclear decommissioning fund available-for-sale securities, unrealized gains	$ 169,600,000		$ 135,800,000
Nuclear decommissioning fund available-for-sale securities, unrealized losses	81,600,000		46,400,000
Available-for-sale Securities [Abstract]
Equity investments in unconsolidated subsidiaries	88,700,000		91,200,000
Miscellaneous investments	38,800,000		37,100,000
Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	1,407,397,000	[1]	1,400,145,000	[2]
Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	1,495,438,000	[1]	1,489,542,000	[2]
Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	546,002,000	[1]	658,201,000	[2]
Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	865,706,000	[1]	726,741,000	[2]
Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	83,730,000	[1]	104,600,000	[2]
Cash equivalents | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	32,663,000	[1]	246,904,000	[2]
Cash equivalents | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	32,663,000	[1]	246,904,000	[2]
Cash equivalents | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	32,663,000	[1]	237,938,000	[2]
Cash equivalents | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	8,966,000	[2]
Cash equivalents | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Commingled funds | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	415,197,000	[1]	396,681,000	[2]
Commingled funds | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	414,899,000	[1]	417,583,000	[2]
Commingled funds | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Commingled funds | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	414,899,000	[1]	417,583,000	[2]
Commingled funds | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
International equity funds | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	66,452,000	[1]	66,452,000	[2]
International equity funds | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	65,606,000	[1]	69,481,000	[2]
International equity funds | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
International equity funds | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	65,606,000	[1]	69,481,000	[2]
International equity funds | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Private equity investments | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	36,496,000	[1]	27,943,000	[2]
Private equity investments | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	45,590,000	[1]	33,250,000	[2]
Private equity investments | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Private equity investments | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Private equity investments | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	45,590,000	[1]	33,250,000	[2]
Real Estate | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Amortized Cost Basis	30,357,000	[1]	32,561,000	[2]
Real Estate | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	38,140,000	[1]	39,074,000	[2]
Real Estate | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Real Estate | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	[1]	0	[2]
Real Estate | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	38,140,000	[1]	39,074,000	[2]
Government Securities | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis	56,017,000	[1]	21,092,000	[2]
Government Securities | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	49,702,000	[1]	21,521,000	[2]
Government Securities | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
Government Securities | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	49,702,000	[1]	21,521,000	[2]
Government Securities | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
U.S. Corporate Bonds | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis	131,917,000	[1]	162,053,000	[2]
U.S. Corporate Bonds | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	134,571,000	[1]	169,488,000	[2]
U.S. Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
U.S. Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	134,571,000	[1]	169,488,000	[2]
U.S. Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
International Corporate Bonds | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis	18,859,000	[1]	15,165,000	[2]
International Corporate Bonds | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	18,703,000	[1]	16,052,000	[2]
International Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
International Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	18,703,000	[1]	16,052,000	[2]
International Corporate Bonds | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
Municipal Bonds | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis	190,353,000	[1]	21,392,000	[2]
Municipal Bonds | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	182,225,000	[1]	23,650,000	[2]
Municipal Bonds | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
Municipal Bonds | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	182,225,000	[1]	23,650,000	[2]
Municipal Bonds | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	[1]	0	[2]
Asset-backed Securities | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis			2,066,000	[2]
Asset-backed Securities | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			2,067,000	[2]
Asset-backed Securities | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			0	[2]
Asset-backed Securities | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			0	[2]
Asset-backed Securities | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			2,067,000	[2]
Mortgage-backed Securities | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Debt Securities, Amortized Cost Basis			28,743,000	[2]
Mortgage-backed Securities | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			30,209,000	[2]
Mortgage-backed Securities | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			0	[2]
Mortgage-backed Securities | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			0	[2]
Mortgage-backed Securities | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities			30,209,000	[2]
Common stock | Nuclear Decommissioning Fund
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Equity Securities, Amortized Cost Basis	429,086,000	[1]	379,093,000	[2]
Common stock | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	513,339,000	[1]	420,263,000	[2]
Common stock | Fair Value Measured on a Recurring Basis | Level 1 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	513,339,000	[1]	420,263,000	[2]
Common stock | Fair Value Measured on a Recurring Basis | Level 2 | Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	0	[1]	0	[2]
Common stock | Fair Value Measured on a Recurring Basis | Level 3 | Nuclear Decommissioning Fund
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 $88.7 million of equity investments in unconsolidated subsidiaries and $38.8 million of miscellaneous investments.

[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $91.2 million of equity investments in unconsolidated subsidiaries and $37.1 million of miscellaneous investments.