Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Gross Unrealized Gain		$ 378,600		$ 328,800
Available-for-sale Securities, Gross Unrealized Loss		46,900		100,200
Investments [Abstract]
Miscellaneous investments		98,300		48,900
Equity investments in unconsolidated subsidiaries		132,800		130,000
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		20,379	[1]	27,484	[2]
Alternative Investments, Fair Value Disclosure		972,591	[1],[3]	884,034	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		1,860,762	[1]	1,724,150	[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 | International equity funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investments, Fair Value Disclosure		245,359	[1],[3]	231,122	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		245,359	[1]	231,122	[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		97,543	[1],[3]	88,467	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		97,543	[1]	88,467	[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		92,091	[1],[3]	77,338	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		92,091	[1]	77,338	[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		190,462	[1],[3]	157,528	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		190,462	[1]	157,528	[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		187,647	[1],[3]	165,190	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		187,647	[1]	165,190	[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		159,489	[1],[3]	164,389	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		159,489	[1]	164,389	[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		31,965	[1]	21,356	[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		105,772	[1]	65,276	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | International 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		21,672	[1]	12,801	[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		13,786	[1]	51,589	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Asset-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]			2,830
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		0	[1],[3]	0	[2],[4]
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Debt Securities		2,816	[1]	11,621	[2]
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		473,400	[1]	432,495	[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		218,381	[1]	214,664	[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		20,379	[1]	27,484	[2]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		712,160	[1]	674,643	[2]
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	[1]	0	[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 | International 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 | Asset-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 | Mortgage-backed 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. equities
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Equity Securities		473,400	[1]	432,495	[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		218,381	[1]	214,664	[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		176,011	[1]	165,473	[2]
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	[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		31,965	[1]	21,356	[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		105,772	[1]	65,276	[2]
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		21,672	[1]	12,801	[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		13,786	[1]	51,589	[2]
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]			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		2,816	[1]	11,621	[2]
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 | International equity 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 | 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 | International 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 | Asset-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 | Mortgage-backed 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. 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]
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		20,379	[1]	27,484	[2]
Available-for-sale Securities [Abstract]
Available-for-sale Securities		1,528,993	[1]	1,495,599	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | International equity funds
Available-for-sale Securities [Abstract]
Available-for-sale Securities		260,877	[1]	259,114	[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		93,597	[1]	88,987	[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]	99,771	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments
Available-for-sale Securities [Abstract]
Available-for-sale Securities		132,190	[1]	105,965	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate
Available-for-sale Securities [Abstract]
Available-for-sale Securities		128,630	[1]	115,019	[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]	150,877	[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		32,764	[1]	24,444	[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		104,913	[1]	73,061	[2]
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		21,751	[1]	13,726	[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		13,609	[1]	49,255	[2]
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]			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		2,785	[1]	11,444	[2]
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		270,779	[1]	273,106	[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		$ 189,100	[1]	$ 200,509	[2]

[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 $98.3 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 No. 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 No. 2015-07.

[4] Based on the requirements of ASU No. 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 No. 2015-07.