Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Cash equivalents		$ 1,200
Commodity contracts,assets	$ 515	1,113
Commodity contracts, liabilities	(69)	(2,029)
Nuclear decommissioning trust	1,010,775	851,134
Nuclear decommissioning trust, other	521,245	398,953
Other special use fund	245,095	236,101
Other special use funds, other	474	593
Total assets	1,256,385	1,089,548
Total assets Other	521,650	397,517
Liabilities
Other	(711)	(875)
Derivative Liability	(72,132)	(60,037)
Equity securities
Assets
Nuclear decommissioning trust	13,273	7,351
Nuclear decommissioning trust, other	2,401	2,148
Other special use fund	7,616	45,723
Other special use funds, other	474	593
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	518,844	396,805
U.S. Treasury debt
Assets
Nuclear decommissioning trust	160,607	148,173
Other special use fund	232,848	173,310
Corporate debt
Assets
Nuclear decommissioning trust	115,869	96,656
Mortgage-backed securities
Assets
Nuclear decommissioning trust	118,795	113,115
Municipal bonds
Assets
Nuclear decommissioning trust	73,040	79,073
Other special use fund	4,631	17,068
Other fixed income
Assets
Nuclear decommissioning trust	10,347	9,961
(Level 1)
Assets
Cash equivalents		1,200
Commodity contracts,assets	0	0
Nuclear decommissioning trust	171,479	153,376
Other special use fund	239,990	218,440
Total assets	411,469	373,016
Liabilities
Gross derivative liability	0	0
(Level 1) \| Equity securities
Assets
Nuclear decommissioning trust	10,872	5,203
Other special use fund	7,142	45,130
(Level 1) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
(Level 1) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	160,607	148,173
Other special use fund	232,848	173,310
(Level 1) \| Corporate debt
Assets
Nuclear decommissioning trust	0	0
(Level 1) \| Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
(Level 1) \| Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 1) \| Other fixed income
Assets
Nuclear decommissioning trust	0	0
(Level 2)
Assets
Cash equivalents		0
Commodity contracts,assets	551	3,140
Nuclear decommissioning trust	318,051	298,805
Other special use fund	4,631	17,068
Total assets	323,233	319,013
Liabilities
Gross derivative liability	(67,992)	(52,696)
(Level 2) \| Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 2) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
(Level 2) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 2) \| Corporate debt
Assets
Nuclear decommissioning trust	115,869	96,656
(Level 2) \| Mortgage-backed securities
Assets
Nuclear decommissioning trust	118,795	113,115
(Level 2) \| Municipal bonds
Assets
Nuclear decommissioning trust	73,040	79,073
Other special use fund	4,631	17,068
(Level 2) \| Other fixed income
Assets
Nuclear decommissioning trust	10,347	9,961
(Level 3)
Assets
Cash equivalents		0
Commodity contracts,assets	33	2
Nuclear decommissioning trust	0	0
Other special use fund	0	0
Total assets	33	2
Liabilities
Gross derivative liability	(3,429)	(8,216)
(Level 3) \| Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 3) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
(Level 3) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 3) \| Corporate debt
Assets
Nuclear decommissioning trust	0	0
(Level 3) \| Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
(Level 3) \| Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use fund	0	0
(Level 3) \| Other fixed income
Assets
Nuclear decommissioning trust	0	0
Fair Value Measured at Net Asset Value Per Share \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	$ 518,844	$ 396,805

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Decommissioning Fund Investments, Fair Value Net Pending Securities Sales and Purchases

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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