Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents		$ 10,630
Commodity contracts, assets	$ 3,316	1,982
Commodity contracts, liabilities	(3,970)	(4,737)
Nuclear decommissioning trust	873,643	871,000
Nuclear decommissioning trust, other	427,199	417,499
Other special use funds	218,540	32,542
Other special use funds, other	1,260	525
Total assets	1,095,499	916,154
Total assets, other	424,489	413,287
Liabilities
Total, other	2,752	1,516
Amount reported on balance sheet	(95,443)	(96,422)
Reclassified assets held for sale	2,000
Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		7,333
Nuclear decommissioning trust, other		109
Equity securities
Assets
Nuclear decommissioning trust	6,666
Nuclear decommissioning trust, other	625
Other special use funds	15,570
Other special use funds, other	1,260
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	426,574	417,390
U.S. Treasury debt
Assets
Nuclear decommissioning trust	137,960	127,662
Other special use funds	178,160
Corporate debt
Assets
Nuclear decommissioning trust	107,225	114,007
Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	107,008	111,874
Municipal bonds
Assets
Nuclear decommissioning trust	79,195	79,049
Other special use funds	24,810
Other fixed income
Assets
Nuclear decommissioning trust	9,015	13,685
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents		10,630
Commodity contracts, assets	0	0
Nuclear decommissioning trust	144,001	134,886
Other special use funds	192,470	455
Total assets	336,471	145,971
Liabilities
Gross derivative liability	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		7,224
Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Assets
Nuclear decommissioning trust	6,041
Other special use funds	14,310
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	137,960	127,662
Other special use funds	178,160
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
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	5,215	5,683
Nuclear decommissioning trust	302,443	318,615
Other special use funds	24,810	31,562
Total assets	332,468	355,860
Liabilities
Gross derivative liability	(86,766)	(78,646)
Significant Other Observable Inputs (Level 2) \| Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		0
Significant Other Observable Inputs (Level 2) \| Equity securities
Assets
Nuclear decommissioning trust	0
Other special use funds	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
Significant Other Observable Inputs (Level 2) \| Corporate debt
Assets
Nuclear decommissioning trust	107,225	114,007
Significant Other Observable Inputs (Level 2) \| Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	107,008	111,874
Significant Other Observable Inputs (Level 2) \| Municipal bonds
Assets
Nuclear decommissioning trust	79,195	79,049
Other special use funds	24,810
Significant Other Observable Inputs (Level 2) \| Other fixed income
Assets
Nuclear decommissioning trust	9,015	13,685
Significant Unobservable Inputs (Level 3)
Assets
Cash equivalents		0
Commodity contracts, assets	2,071	1,036
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	2,071	1,036
Liabilities
Gross derivative liability	(11,429)	(19,292)
Amount reported on balance sheet	(11,429)	(19,292)
Significant Unobservable Inputs (Level 3) \| Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		0
Significant Unobservable Inputs (Level 3) \| Equity securities
Assets
Nuclear decommissioning trust	0
Other special use funds	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
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
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	$ 426,574	$ 417,390

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, Reclassified Assets Held For Sale

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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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