Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Gross Unrealized Gain	$ 328,800	$ 312,100
Available-for-sale Securities, Gross Unrealized Loss	100,200	74,100
Investments [Abstract]
Equity investments in unconsolidated subsidiaries	130,000	83,100
Miscellaneous investments	48,900	45,600
Nuclear Decommissioning Fund
Available-for-sale Securities [Abstract]
Available-for-sale Securities	1,724,150	1,703,921
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,484	24,184
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Commingled funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	410,634	465,615
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	231,122	78,721
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities	157,528	101,237
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities	84,750	64,249
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Government securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	21,356	28,808
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | U.S. Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	65,276	77,562
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | International Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	12,801	16,341
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Municipal Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	51,589	249,201
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Asset-backed Securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	2,830	9,250
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Mortgage-backed securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	11,621	23,895
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Common stock
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	647,159	564,858
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,484	24,184
Available-for-sale Securities [Abstract]
Available-for-sale Securities	674,643	589,042
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1 | Commingled funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1 | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1 | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1 | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 1 | Government securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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, Measurements, Recurring | 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, Measurements, Recurring | Fair Value | Level 1 | Municipal Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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, Measurements, Recurring | 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, Measurements, Recurring | Fair Value | Level 1 | Common stock
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	647,159	564,858
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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	807,229	949,393
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Commingled funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	410,634	465,615
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	231,122	78,721
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Government securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	21,356	28,808
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | U.S. Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	65,276	77,562
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | International Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	12,801	16,341
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Municipal Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	51,589	249,201
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Asset-backed Securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	2,830	9,250
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Mortgage-backed securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	11,621	23,895
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 2 | Common stock
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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	242,278	165,486
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 3 | Commingled funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 3 | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 3 | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities	157,528	101,237
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 3 | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities	84,750	64,249
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Fair Value | Level 3 | Government securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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, Measurements, Recurring | 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, Measurements, Recurring | Fair Value | Level 3 | Municipal Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | 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, Measurements, Recurring | 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, Measurements, Recurring | Fair Value | Level 3 | Common stock
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	0	0
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	27,484	24,184
Available-for-sale Securities [Abstract]
Available-for-sale Securities	1,495,599	1,465,922
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Commingled funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	392,838	470,013
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities	259,114	80,454
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities	105,965	73,936
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities	61,816	43,859
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Government securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	24,444	30,674
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | U.S. Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	73,061	81,463
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | International Corporate Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	13,726	16,950
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Municipal Bonds
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	49,255	242,282
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Asset-backed Securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	2,837	9,131
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Mortgage-backed securities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities	11,444	23,225
Nuclear Decommissioning Fund | Fair Value, Measurements, Recurring | Cost | Common stock
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities	$ 473,615	$ 369,751