Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Cash equivalents		$ 1,200
Commodity contracts, assets	$ 898	1,113
Commodity contracts, liabilities	(3,776)	(2,029)
Nuclear decommissioning trust	919,309	851,134
Nuclear decommissioning trust, other	453,859	398,953
Other special use funds	238,207	236,101
Other special use funds, other	1,563	593
Total assets	1,158,414	1,089,548
Total assets, other	451,646	397,517
Liabilities
Total, other	2,079	875
Amount reported on balance sheet	(48,133)	(60,037)
Equity securities
Assets
Nuclear decommissioning trust	10,177	7,351
Nuclear decommissioning trust, other	2,630	2,148
Other special use funds	13,034	45,723
Other special use funds, other	1,563	593
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	451,229	396,805
U.S. Treasury debt
Assets
Nuclear decommissioning trust	155,231	148,173
Other special use funds	208,945	173,310
Corporate debt
Assets
Nuclear decommissioning trust	108,509	96,656
Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	113,002	113,115
Municipal bonds
Assets
Nuclear decommissioning trust	70,375	79,073
Other special use funds	16,228	17,068
Other fixed income
Assets
Nuclear decommissioning trust	10,786	9,961
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents		1,200
Commodity contracts, assets	0	0
Nuclear decommissioning trust	162,778	153,376
Other special use funds	220,416	218,440
Total assets	383,194	373,016
Liabilities
Gross derivative liability	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Assets
Nuclear decommissioning trust	7,547	5,203
Other special use funds	11,471	45,130
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	155,231	148,173
Other special use funds	208,945	173,310
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Corporate debt
Assets
Nuclear decommissioning trust	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Other fixed income
Assets
Nuclear decommissioning trust	0	0
Significant Other Observable Inputs (Level 2)
Assets
Cash equivalents		0
Commodity contracts, assets	3,598	3,140
Nuclear decommissioning trust	302,672	298,805
Other special use funds	16,228	17,068
Total assets	322,498	319,013
Liabilities
Gross derivative liability	(43,524)	(52,696)
Significant Other Observable Inputs (Level 2) \| Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Significant Other Observable Inputs (Level 2) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Significant Other Observable Inputs (Level 2) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Significant Other Observable Inputs (Level 2) \| Corporate debt
Assets
Nuclear decommissioning trust	108,509	96,656
Significant Other Observable Inputs (Level 2) \| Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	113,002	113,115
Significant Other Observable Inputs (Level 2) \| Municipal bonds
Assets
Nuclear decommissioning trust	70,375	79,073
Other special use funds	16,228	17,068
Significant Other Observable Inputs (Level 2) \| Other fixed income
Assets
Nuclear decommissioning trust	10,786	9,961
Significant Unobservable Inputs (Level 3)
Assets
Cash equivalents		0
Commodity contracts, assets	1,076	2
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	1,076	2
Liabilities
Gross derivative liability	(6,688)	(8,216)
Significant Unobservable Inputs (Level 3) \| Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Significant Unobservable Inputs (Level 3) \| U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Significant Unobservable Inputs (Level 3) \| U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Significant Unobservable Inputs (Level 3) \| Corporate debt
Assets
Nuclear decommissioning trust	0	0
Significant Unobservable Inputs (Level 3) \| Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	0	0
Significant Unobservable Inputs (Level 3) \| Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Significant Unobservable Inputs (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	$ 451,229	$ 396,805

Assets, Fair Value, Gross Liability and Net Pending Securities Sales and Purchases

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

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Special Use Fund Investments, Other, Gross Fair Value

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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 portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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The fair value of investments held in a trust fund to pay for the costs of decontaminating and decommissioning facilities, whether such amount is presented as a separate caption or as a parenthetical disclosure on the balance sheet. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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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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