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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 751,954 | $ 735,196
Total assets | 23,847 | 30,364
Recurring |  |
Assets |  |
Derivative instruments, other | (11,759) | (25,345)
Derivative assets | 24,701 | 28,011
Nuclear decommissioning trust, other | 318,073 | 314,622
Nuclear decommissioning trust | 751,954 | 735,196
Total, other | 306,314 | 289,277
Total assets | 776,655 | 763,207
Liabilities |  |
Total, other | 25,022 | 39,698
Derivative liability | (178,886) | (167,689)
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust, other | 319,971 | 314,957
Nuclear decommissioning trust | 319,971 | 314,957
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other | (1,898) | (335)
Nuclear decommissioning trust | 9,637 | 11,925
Recurring | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 107,698 | 117,245
Recurring | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 101,687 | 96,243
Recurring | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 109,298 | 99,065
Recurring | Municipality bonds |  |
Assets |  |
Nuclear decommissioning trust | 80,344 | 72,206
Recurring | Other |  |
Assets |  |
Nuclear decommissioning trust | 23,319 | 23,555
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Decommissioning fund investments, gross fair value | 119,233 | 129,505
Gross assets, fair value disclosure | 119,233 | 129,505
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 11,535 | 12,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 107,698 | 117,245
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 12,613 | 22,992
Decommissioning fund investments, gross fair value | 314,648 | 291,069
Gross assets, fair value disclosure | 327,261 | 314,061
Liabilities |  |
Gross derivative liability | (140,554) | (144,044)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 101,687 | 96,243
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 109,298 | 99,065
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 80,344 | 72,206
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 23,319 | 23,555
Recurring | Significant Unobservable Inputs (a) (Level 3) |  |
Assets |  |
Gross derivative assets | 23,847 | 30,364
Gross assets, fair value disclosure | 23,847 | 30,364
Liabilities |  |
Gross derivative liability | $ (63,354) | $ (63,343)

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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