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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Assets |  |
Cash equivalents |  | $ 1,200
Commodity contracts, assets | $ 535 | 1,113
Commodity contracts, liabilities | (3,099) | (2,029)
Nuclear decommissioning trust | 950,448 | 851,134
Nuclear decommissioning trust, other | 476,040 | 398,953
Other special use funds | 241,558 | 236,101
Other special use funds, other | 997 | 593
Total assets | 1,192,541 | 1,089,548
Total assets, other | 473,938 | 397,517
Liabilities |  |
Total, other | 1,755 | 875
Amount reported on balance sheet | (80,426) | (60,037)
Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 11,984 | 7,351
Nuclear decommissioning trust, other | 6,665 | 2,148
Other special use funds | 14,197 | 45,723
Other special use funds, other | 997 | 593
U.S. commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust | 469,375 | 396,805
U.S. Treasury debt |  |
Assets |  |
Nuclear decommissioning trust | 162,044 | 148,173
Other special use funds | 214,712 | 173,310
Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 115,674 | 96,656
Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 113,179 | 113,115
Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 67,158 | 79,073
Other special use funds | 12,649 | 17,068
Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 11,034 | 9,961
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Cash equivalents |  | 1,200
Commodity contracts, assets | 0 | 0
Nuclear decommissioning trust | 167,363 | 153,376
Other special use funds | 227,912 | 218,440
Total assets | 395,275 | 373,016
Liabilities |  |
Gross derivative liability | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 5,319 | 5,203
Other special use funds | 13,200 | 45,130
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 | 162,044 | 148,173
Other special use funds | 214,712 | 173,310
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 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 | 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 | 3,198 | 3,140
Nuclear decommissioning trust | 307,045 | 298,805
Other special use funds | 12,649 | 17,068
Total assets | 322,892 | 319,013
Liabilities |  |
Gross derivative liability | (68,992) | (52,696)
Significant Other Observable Inputs (Level 2) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 | 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 | 0
Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 115,674 | 96,656
Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 113,179 | 113,115
Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 67,158 | 79,073
Other special use funds | 12,649 | 17,068
Significant Other Observable Inputs (Level 2) | Other fixed income |  |
Assets |  |
Nuclear decommissioning trust | 11,034 | 9,961
Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Cash equivalents |  | 0
Commodity contracts, assets | 436 | 2
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 | 0
Total assets | 436 | 2
Liabilities |  |
Gross derivative liability | (13,189) | (8,216)
Significant Unobservable Inputs (Level 3) | Equity securities |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Other special use funds | 0 | 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 | 0
Significant Unobservable Inputs (Level 3) | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 0 | 0
Significant Unobservable Inputs (Level 3) | Mortgage-backed 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 | 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 | $ 469,375 | $ 396,805

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