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Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 871,000 | $ 779,586
Total assets | 1,036 | 11,076
Fair value measurement on a recurring basis |  |
Assets |  |
Cash equivalents | 10,630 |
Coal reclamation escrow account | 32,542 | 14,521
Coal reclamation escrow account, other | 525 |
Other | (4,737) | (35,103)
Derivative assets | 1,982 | 19,695
Other | 417,499 | 354,056
Nuclear decommissioning trust | 871,000 | 779,586
Other | 413,287 | 318,953
Total assets | 916,154 | 813,802
Liabilities |  |
Other | 1,516 | 31,049
Derivative Liability | (96,422) | (73,074)
Fair value measurement on a recurring basis | Cash and cash equivalents |  |
Assets |  |
Other | 109 | 795
Nuclear decommissioning trust | 7,333 | 795
Fair value measurement on a recurring basis | US commingled equity funds |  |
Assets |  |
Other | 417,390 | 353,261
Nuclear decommissioning trust | 417,390 | 353,261
Fair value measurement on a recurring basis | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 127,662 | 95,441
Fair value measurement on a recurring basis | Corporate |  |
Assets |  |
Nuclear decommissioning trust | 114,007 | 111,623
Fair value measurement on a recurring basis | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 111,874 | 115,337
Fair value measurement on a recurring basis | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 79,049 | 80,997
Fair value measurement on a recurring basis | Other |  |
Assets |  |
Nuclear decommissioning trust | 13,685 | 22,132
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Cash equivalents | 10,630 |
Coal reclamation escrow account | 455 | 14,521
Decommissioning fund investments, gross fair value | 134,886 | 95,441
Gross assets, fair value disclosure | 145,971 | 109,962
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 |  | 0
Nuclear decommissioning trust | 7,224 |
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 | 127,662 | 95,441
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 5,683 | 43,722
Coal reclamation escrow account | 31,562 |
Decommissioning fund investments, gross fair value | 318,615 | 330,089
Gross assets, fair value disclosure | 355,860 | 373,811
Liabilities |  |
Gross derivative liability | (78,646) | (45,641)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate |  |
Assets |  |
Decommissioning fund investments, gross fair value | 114,007 | 111,623
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 111,874 | 115,337
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 79,049 | 80,997
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 13,685 | 22,132
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 1,036 | 11,076
Gross assets, fair value disclosure | 1,036 | 11,076
Liabilities |  |
Gross derivative liability | $ (19,292) | $ (58,482)

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- Definition Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258 + Details
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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 No definition available. + 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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- Definition Fair value, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + Details
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