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Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 779,586 | $ 735,196
Total assets | 11,076 | 30,364
Fair value measurement on a recurring basis |  |
Assets |  |
Coal reclamation trust - Cash equivalents | 14,521 |
Other | (35,103) | (25,345)
Derivative assets | 19,695 | 28,011
Other | 354,056 | 314,622
Nuclear decommissioning trust | 779,586 | 735,196
Other | 318,953 | 289,277
Total assets | 813,802 | 763,207
Liabilities |  |
Other | 31,049 | 39,698
Derivative Liability | (73,074) | (167,689)
Fair value measurement on a recurring basis | US commingled equity funds |  |
Assets |  |
Other | 353,261 | 314,957
Nuclear decommissioning trust | 353,261 | 314,957
Fair value measurement on a recurring basis | Cash and cash equivalents |  |
Assets |  |
Other | 795 | (335)
Nuclear decommissioning trust | 795 | 11,925
Fair value measurement on a recurring basis | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 95,441 | 117,245
Fair value measurement on a recurring basis | Corporate |  |
Assets |  |
Nuclear decommissioning trust | 111,623 | 96,243
Fair value measurement on a recurring basis | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 115,337 | 99,065
Fair value measurement on a recurring basis | Municipality bonds |  |
Assets |  |
Nuclear decommissioning trust | 80,997 | 72,206
Fair value measurement on a recurring basis | Other |  |
Assets |  |
Nuclear decommissioning trust | 22,132 | 23,555
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Coal reclamation trust - Cash equivalents | 14,521 |
Decommissioning fund investments, gross fair value | 95,441 | 129,505
Gross assets, fair value disclosure | 109,962 | 129,505
Liabilities |  |
Gross derivative liability | 0 | 0
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents |  |
Assets |  |
Decommissioning fund investments, gross fair value |  | 12,260
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 95,441 | 117,245
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 43,722 | 22,992
Decommissioning fund investments, gross fair value | 330,089 | 291,069
Gross assets, fair value disclosure | 373,811 | 314,061
Liabilities |  |
Gross derivative liability | (45,641) | (144,044)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate |  |
Assets |  |
Decommissioning fund investments, gross fair value | 111,623 | 96,243
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 115,337 | 99,065
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 80,997 | 72,206
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 22,132 | 23,555
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 11,076 | 30,364
Gross assets, fair value disclosure | 11,076 | 30,364
Liabilities |  |
Gross derivative liability | $ (58,482) | $ (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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