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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Assets |  |
Total | $ 841,980 | $ 779,586
Total assets | 3,472 | 11,076
Municipal bonds |  |
Assets |  |
Coal reclamation escrow amount | 24,973 |
Recurring |  |
Assets |  |
Coal reclamation escrow amount |  | 14,521
Derivative instruments, other | (9,851) | (35,103)
Derivative assets | 2,050 | 19,695
Nuclear decommissioning trust, other | 406,291 | 354,056
Total | 841,980 | 779,586
Total, other | 396,950 | 318,953
Total assets | 876,688 | 813,802
Liabilities |  |
Total, other | 9,247 | 31,049
Amount Reported on Balance Sheet | (86,244) | (73,074)
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust, other | 401,913 | 353,261
Total | 401,913 | 353,261
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other | 4,378 | 795
Total | 14,976 | 795
Recurring | U.S. Treasury |  |
Assets |  |
Total | 90,776 | 95,441
Recurring | Corporate debt |  |
Assets |  |
Total | 124,369 | 111,623
Recurring | Mortgage-backed securities |  |
Assets |  |
Total | 116,237 | 115,337
Recurring | Municipal bonds |  |
Assets |  |
Total | 76,412 | 80,997
Recurring | Other |  |
Assets |  |
Total | 17,297 | 22,132
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Coal reclamation escrow amount |  | 14,521
Decommissioning fund investments, gross fair value | 101,374 | 95,441
Gross assets, fair value disclosure | 108,549 | 109,962
Liabilities |  |
Gross derivative liability | 0 | 0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 10,598 | 0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 90,776 | 95,441
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 8,429 | 43,722
Decommissioning fund investments, gross fair value | 334,315 | 330,089
Gross assets, fair value disclosure | 367,717 | 373,811
Liabilities |  |
Gross derivative liability | (53,414) | (45,641)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 124,369 | 111,623
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 116,237 | 115,337
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Coal reclamation escrow amount | 24,973 |
Decommissioning fund investments, gross fair value | 76,412 | 80,997
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 17,297 | 22,132
Recurring | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 3,472 | 11,076
Gross assets, fair value disclosure | 3,472 | 11,076
Liabilities |  |
Gross derivative liability | (42,077) | $ (58,482)
Cash equivalents | Recurring |  |
Assets |  |
Coal reclamation escrow amount | 7,685 |
Coal reclamation escrow amount, other | 510 |
Cash equivalents | Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Coal reclamation escrow amount | $ 7,175 |

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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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