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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2016 | Dec. 31, 2015
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 767,416 | $ 735,196
Total assets | 18,118 | 30,364
Recurring |  |
Assets |  |
Cash equivalents | 9,857 |
Derivative instruments, other | (22,487) | (25,345)
Derivative assets | 22,140 | 28,011
Nuclear decommissioning trust, other | 314,898 | 314,622
Nuclear decommissioning trust | 767,416 | 735,196
Total, other | 292,411 | 289,277
Total assets | 799,413 | 763,207
Liabilities |  |
Total, other | 18,864 | 39,698
Derivative liability | (107,550) | (167,689)
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust, other | 328,037 | 314,957
Nuclear decommissioning trust | 328,037 | 314,957
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other | (13,139) | (335)
Nuclear decommissioning trust | 4,753 | 11,925
Recurring | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 117,448 | 117,245
Recurring | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 106,399 | 96,243
Recurring | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 112,771 | 99,065
Recurring | Municipality bonds |  |
Assets |  |
Nuclear decommissioning trust | 73,847 | 72,206
Recurring | Other |  |
Assets |  |
Nuclear decommissioning trust | 24,161 | 23,555
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Cash equivalents | 9,857 |
Decommissioning fund investments, gross fair value | 135,340 | 129,505
Gross assets, fair value disclosure | 145,197 | 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 | 17,892 | 12,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 117,448 | 117,245
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 26,509 | 22,992
Decommissioning fund investments, gross fair value | 317,178 | 291,069
Gross assets, fair value disclosure | 343,687 | 314,061
Liabilities |  |
Gross derivative liability | (75,916) | (144,044)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 106,399 | 96,243
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 112,771 | 99,065
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 73,847 | 72,206
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 24,161 | 23,555
Recurring | Significant Unobservable Inputs (a) (Level 3) |  |
Assets |  |
Gross derivative assets | 18,118 | 30,364
Gross assets, fair value disclosure | 18,118 | 30,364
Liabilities |  |
Gross derivative liability | $ (50,498) | $ (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 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + 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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