Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 5,600	$ 10,630
Commodity contracts, assets	1,224	1,982
Commodity contracts, liabilities	(1,699)	(4,737)
Nuclear decommissioning trust	906,687	871,000
Nuclear decommissioning trust, other	459,165	417,499
Other special use funds	233,740	32,542
Other special use funds, other	1,722	525
Total assets	1,147,251	916,154
Total assets, other	459,188	413,287
Liabilities
Total, other	48	1,516
Amount reported on balance sheet	(79,730)	(96,422)
Cash and cash equivalent funds
Assets
Nuclear decommissioning trust		7,333
Nuclear decommissioning trust, other		109
Equity securities
Assets
Nuclear decommissioning trust	5,588
Nuclear decommissioning trust, other	(625)
Other special use funds	13,755
Other special use funds, other	1,722
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	459,790	417,390
U.S. Treasury debt
Assets
Nuclear decommissioning trust	134,462	127,662
Other special use funds	199,094
Corporate debt
Assets
Nuclear decommissioning trust	104,953	114,007
Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	112,036	111,874
Municipal bonds
Assets
Nuclear decommissioning trust	80,787	79,049
Other special use funds	20,891
Other fixed income
Assets
Nuclear decommissioning trust	9,071	13,685
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents	5,600	10,630
Commodity contracts, assets	0	0
Nuclear decommissioning trust	140,675	134,886
Other special use funds	211,127	455
Total assets	357,402	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	6,213
Other special use funds	12,033
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	134,462	127,662
Other special use funds	199,094
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	2,865	5,683
Nuclear decommissioning trust	306,847	318,615
Other special use funds	20,891	31,562
Total assets	330,603	355,860
Liabilities
Gross derivative liability	(69,857)	(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	104,953	114,007
Significant Other Observable Inputs (Level 2) \| Mortgage-backed debt securities
Assets
Nuclear decommissioning trust	112,036	111,874
Significant Other Observable Inputs (Level 2) \| Municipal bonds
Assets
Nuclear decommissioning trust	80,787	79,049
Other special use funds	20,891
Significant Other Observable Inputs (Level 2) \| Other fixed income
Assets
Nuclear decommissioning trust	9,071	13,685
Significant Unobservable Inputs (Level 3)
Assets
Cash equivalents		0
Commodity contracts, assets	58	1,036
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	58	1,036
Liabilities
Gross derivative liability	(9,921)	(19,292)
Amount reported on balance sheet	(9,921)	(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	$ 459,790	$ 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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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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