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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 723,582 | $ 713,866
Total assets | 30,000 | 33,000
Recurring |  |
Assets |  |
Derivative instruments, other | (13,000) | (23,000)
Derivative assets | 33,000 | 31,000
Nuclear decommissioning trust, other | (4,000) | (7,000)
Nuclear decommissioning trust | 724,000 | 714,000
Total, other | (17,000) | (30,000)
Total assets | 757,000 | 745,000
Liabilities |  |
Derivative instruments, other | 28,000 | 59,000
Derivative liability | (148,000) | (110,000)
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 314,000 | 310,000
Recurring | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 95,000 | 119,000
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other | (4,000) | (7,000)
Nuclear decommissioning trust | 7,000 | 4,000
Recurring | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 116,000 | 109,000
Recurring | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 87,000 | 89,000
Recurring | Municipality bonds |  |
Assets |  |
Nuclear decommissioning trust | 86,000 | 69,000
Recurring | Other |  |
Assets |  |
Nuclear decommissioning trust | 19,000 | 14,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Decommissioning fund investments, gross fair value | 93,000 | 119,000
Gross assets, fair value disclosure | 93,000 | 119,000
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 93,000 | 119,000
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 16,000 | 21,000
Decommissioning fund investments, gross fair value | 635,000 | 602,000
Gross assets, fair value disclosure | 651,000 | 623,000
Liabilities |  |
Gross derivative liability | (103,000) | (95,000)
Recurring | Significant Other Observable Inputs (Level 2) | US commingled equity funds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 314,000 | 310,000
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 2,000 |
Recurring | Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 11,000 | 11,000
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 116,000 | 109,000
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 87,000 | 89,000
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 86,000 | 69,000
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 19,000 | 14,000
Recurring | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 30,000 | 33,000
Gross assets, fair value disclosure | 30,000 | 33,000
Liabilities |  |
Gross derivative liability | $ (73,000) | $ (74,000)

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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=36462937&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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