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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Assets |  |
Total | $ 822,244 | $ 779,586
Total assets | 6,440 | 11,076
Municipal bonds |  |
Assets |  |
Coal reclamation escrow amount | 14,839 |
Recurring |  |
Assets |  |
Coal reclamation escrow amount |  | 14,521
Derivative instruments, other | (16,882) | (35,103)
Derivative assets | 362 | 19,695
Nuclear decommissioning trust, other | 386,832 | 354,056
Total | 822,244 | 779,586
Total, other | 369,950 | 318,953
Total assets | 837,445 | 813,802
Liabilities |  |
Total, other | 17,598 | 31,049
Amount Reported on Balance Sheet | (95,199) | (73,074)
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust, other | 384,999 | 353,261
Total | 384,999 | 353,261
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other | 1,833 | 795
Total | 1,833 | 795
Recurring | U.S. Treasury |  |
Assets |  |
Total | 101,337 | 95,441
Recurring | Corporate debt |  |
Assets |  |
Total | 120,288 | 111,623
Recurring | Mortgage-backed securities |  |
Assets |  |
Total | 113,950 | 115,337
Recurring | Municipal bonds |  |
Assets |  |
Total | 80,659 | 80,997
Recurring | Other |  |
Assets |  |
Total | 19,178 | 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 | 101,337 | 95,441
Gross assets, fair value disclosure | 101,337 | 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 | 0 | 0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 101,337 | 95,441
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 10,804 | 43,722
Decommissioning fund investments, gross fair value | 334,075 | 330,089
Gross assets, fair value disclosure | 359,718 | 373,811
Liabilities |  |
Gross derivative liability | (70,112) | (45,641)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 120,288 | 111,623
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 113,950 | 115,337
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Coal reclamation escrow amount | 14,839 |
Decommissioning fund investments, gross fair value | 80,659 | 80,997
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 19,178 | 22,132
Recurring | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 6,440 | 11,076
Gross assets, fair value disclosure | 6,440 | 11,076
Liabilities |  |
Gross derivative liability | $ (42,685) | $ (58,482)

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