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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Nuclear decommissioning trust | $ 861,439 | $ 871,000
Total assets | 1,548 | 1,036
Recurring |  |
Assets |  |
Cash equivalents |  | 10,630
Derivative instruments, other | (4,809) | (4,737)
Derivative assets | 1,994 | 1,982
Nuclear decommissioning trust, other | 413,623 | 417,499
Nuclear decommissioning trust | 861,439 | 871,000
Other special use funds, net purchases and sales | 1,482 | 525
Other special use funds | 217,992 | 32,542
Total Assets, Other | 410,296 | 413,287
Total assets | 1,081,425 | 916,154
Liabilities |  |
Total, other | 3,075 | 1,516
Amount Reported on Balance Sheet | (115,369) | (96,422)
Recurring | Equity Securities |  |
Assets |  |
Nuclear decommissioning trust, other | (67) |
Nuclear decommissioning trust | 6,508 |
Other special use funds, net purchases and sales | 1,482 |
Other special use funds | 16,445 |
Recurring | US commingled equity funds |  |
Assets |  |
Nuclear decommissioning trust, other | 413,690 | 417,390
Nuclear decommissioning trust | 413,690 | 417,390
Recurring | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust, other |  | 109
Nuclear decommissioning trust |  | 7,333
Recurring | U.S. Treasury |  |
Assets |  |
Nuclear decommissioning trust | 128,396 | 127,662
Other special use funds, net purchases and sales | 0 |
Other special use funds | 174,111 |
Recurring | Corporate debt |  |
Assets |  |
Nuclear decommissioning trust | 111,735 | 114,007
Recurring | Mortgage-backed securities |  |
Assets |  |
Nuclear decommissioning trust | 108,951 | 111,874
Recurring | Municipal bonds |  |
Assets |  |
Nuclear decommissioning trust | 80,604 | 79,049
Other special use funds | 27,436 |
Recurring | Other |  |
Assets |  |
Nuclear decommissioning trust | 11,555 | 13,685
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Assets |  |
Cash equivalents |  | 10,630
Decommissioning fund investments, gross fair value | 134,971 | 134,886
Other special use funds, gross | 189,074 | 455
Gross assets, fair value disclosure | 324,045 | 145,971
Liabilities |  |
Gross derivative liability | 0 | 0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity Securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 6,575 |
Other special use funds, gross | 14,963 |
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds |  |
Assets |  |
Nuclear decommissioning trust |  | 7,224
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury |  |
Assets |  |
Decommissioning fund investments, gross fair value | 128,396 | 127,662
Other special use funds, gross | 174,111 |
Recurring | Significant Other Observable Inputs (Level 2) |  |
Assets |  |
Gross derivative assets | 5,255 | 5,683
Decommissioning fund investments, gross fair value | 312,845 | 318,615
Other special use funds, gross | 27,436 | 31,562
Gross assets, fair value disclosure | 345,536 | 355,860
Liabilities |  |
Gross derivative liability | (97,142) | (78,646)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt |  |
Assets |  |
Decommissioning fund investments, gross fair value | 111,735 | 114,007
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities |  |
Assets |  |
Decommissioning fund investments, gross fair value | 108,951 | 111,874
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds |  |
Assets |  |
Decommissioning fund investments, gross fair value | 80,604 | 79,049
Other special use funds, gross | 27,436 |
Recurring | Significant Other Observable Inputs (Level 2) | Other |  |
Assets |  |
Decommissioning fund investments, gross fair value | 11,555 | 13,685
Recurring | Significant Unobservable Inputs (Level 3) |  |
Assets |  |
Gross derivative assets | 1,548 | 1,036
Gross assets, fair value disclosure | 1,548 | 1,036
Liabilities |  |
Gross derivative liability | $ (21,302) | $ (19,292)

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