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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Cash equivalents |  | $ 10,630
Commodity contracts, assets | $ 3,316 | 1,982
Commodity contracts, liabilities | (3,970) | (4,737)
Nuclear decommissioning trust | 873,643 | 871,000
Nuclear decommissioning trust, other | 427,199 | 417,499
Other special use funds | 218,540 | 32,542
Other special use funds, other | 1,260 | 525
Total assets | 1,095,499 | 916,154
Total assets, other | 424,489 | 413,287
Liabilities |  |
Total, other | 2,752 | 1,516
Amount reported on balance sheet | (95,443) | (96,422)
Reclassified assets held for sale | 2,000 |
Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust |  | 7,333
Nuclear decommissioning trust, other |  | 109
Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 6,666 |
Nuclear decommissioning trust, other | 625 |
Other special use funds | 15,570 |
Other special use funds, other | 1,260 |
U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 426,574 | 417,390
U.S. Treasury debt |  |
Assets |  |
Nuclear decommissioning trust | 137,960 | 127,662
Other special use funds | 178,160 |
Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 107,225 | 114,007
Mortgage-backed debt securities |  |
Assets |  |
Nuclear decommissioning trust | 107,008 | 111,874
Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 79,195 | 79,049
Other special use funds | 24,810 |
Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 9,015 | 13,685
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Cash equivalents |  | 10,630
Commodity contracts, assets | 0 | 0
Nuclear decommissioning trust | 144,001 | 134,886
Other special use funds | 192,470 | 455
Total assets | 336,471 | 145,971
Liabilities |  |
Gross derivative liability | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust |  | 7,224
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 6,041 |
Other special use funds | 14,310 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury debt |  |
Assets |  |
Nuclear decommissioning trust | 137,960 | 127,662
Other special use funds | 178,160 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-backed debt securities |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Cash equivalents |  | 0
Commodity contracts, assets | 5,215 | 5,683
Nuclear decommissioning trust | 302,443 | 318,615
Other special use funds | 24,810 | 31,562
Total assets | 332,468 | 355,860
Liabilities |  |
Gross derivative liability | (86,766) | (78,646)
Significant Other Observable Inputs (Level 2) | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust |  | 0
Significant Other Observable Inputs (Level 2) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 0 |
Other special use funds | 0 |
Significant Other Observable Inputs (Level 2) | U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Other Observable Inputs (Level 2) | U.S. Treasury debt |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 |
Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 107,225 | 114,007
Significant Other Observable Inputs (Level 2) | Mortgage-backed debt securities |  |
Assets |  |
Nuclear decommissioning trust | 107,008 | 111,874
Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 79,195 | 79,049
Other special use funds | 24,810 |
Significant Other Observable Inputs (Level 2) | Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 9,015 | 13,685
Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Cash equivalents |  | 0
Commodity contracts, assets | 2,071 | 1,036
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 | 0
Total assets | 2,071 | 1,036
Liabilities |  |
Gross derivative liability | (11,429) | (19,292)
Amount reported on balance sheet | (11,429) | (19,292)
Significant Unobservable Inputs (Level 3) | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust |  | 0
Significant Unobservable Inputs (Level 3) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 0 |
Other special use funds | 0 |
Significant Unobservable Inputs (Level 3) | U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Unobservable Inputs (Level 3) | U.S. Treasury debt |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 |
Significant Unobservable Inputs (Level 3) | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Unobservable Inputs (Level 3) | Mortgage-backed debt securities |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Unobservable Inputs (Level 3) | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 |
Significant Unobservable Inputs (Level 3) | Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Fair Value Measured at Net Asset Value Per Share | U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | $ 426,574 | $ 417,390

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