v3.25.2
Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2025
Dec. 31, 2024
Fair value    
Nuclear decommissioning trust fund $ 784,480 $ 721,624
Long-term investments 625,900 645,166
Short-term investments 90,269 124,572
Recurring basis | Natural gas swaps    
Fair value    
Derivative liabilities 36,027 28,624
Recurring basis | Domestic equity    
Fair value    
Nuclear decommissioning trust fund 260,203 245,313
Recurring basis | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust fund 90,658 82,316
Long-term investments 23,346 20,972
Recurring basis | US Treasury securities    
Fair value    
Nuclear decommissioning trust fund 65,285 53,806
Long-term investments 29,616 25,654
Recurring basis | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust fund 65,726 70,193
Long-term investments 20,573 20,232
Recurring basis | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust fund 101,986 95,175
Long-term investments 386,250 394,595
Recurring basis | Municipal bonds    
Fair value    
Nuclear decommissioning trust fund 9,542 3,375
Recurring basis | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 9,099 8,487
Recurring basis | Treasury STRIPS    
Fair value    
Long-term investments 114,622 142,199
Short-term investments 90,269 124,572
Recurring basis | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 6,237 3,319
Long-term investments 3,119 1,805
Recurring basis | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 172,645 4,228
Long-term investments 47,659 39,340
Recurring basis | Money Market    
Fair value    
Nuclear decommissioning trust fund   138,553
Recurring basis | Other    
Fair value    
Nuclear decommissioning trust fund 3,099 16,859
Long-term investments 715 369
Recurring basis | (Level 1) | Natural gas swaps    
Fair value    
Derivative liabilities 0 0
Recurring basis | (Level 1) | Domestic equity    
Fair value    
Nuclear decommissioning trust fund 260,203 245,313
Recurring basis | (Level 1) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 1) | US Treasury securities    
Fair value    
Nuclear decommissioning trust fund 65,285 53,806
Long-term investments 29,616 25,654
Recurring basis | (Level 1) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 1) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust fund 101,986 95,175
Long-term investments 386,250 394,595
Recurring basis | (Level 1) | Municipal bonds    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 1) | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 1) | Treasury STRIPS    
Fair value    
Long-term investments 0 0
Short-term investments 0 0
Recurring basis | (Level 1) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 1) | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 1) | Money Market    
Fair value    
Nuclear decommissioning trust fund   138,553
Recurring basis | (Level 1) | Other    
Fair value    
Nuclear decommissioning trust fund 3,099 16,859
Long-term investments 715 369
Recurring basis | (Level 2) | Natural gas swaps    
Fair value    
Derivative liabilities 36,027 28,624
Recurring basis | (Level 2) | Domestic equity    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 2) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust fund 90,138 82,309
Long-term investments 23,346 20,972
Recurring basis | (Level 2) | US Treasury securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 2) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust fund 65,726 70,193
Long-term investments 20,573 20,232
Recurring basis | (Level 2) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 2) | Municipal bonds    
Fair value    
Nuclear decommissioning trust fund 9,542 3,375
Recurring basis | (Level 2) | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 9,099 8,487
Recurring basis | (Level 2) | Treasury STRIPS    
Fair value    
Long-term investments 114,622 142,199
Short-term investments 90,269 124,572
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 6,237 3,319
Long-term investments 3,119 1,805
Recurring basis | (Level 2) | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 172,645 4,228
Long-term investments 47,659 39,340
Recurring basis | (Level 2) | Money Market    
Fair value    
Nuclear decommissioning trust fund   0
Recurring basis | (Level 2) | Other    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | Natural gas swaps    
Fair value    
Derivative liabilities 0 0
Recurring basis | (Level 3) | Domestic equity    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 3) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust fund 520 7
Long-term investments 0 0
Recurring basis | (Level 3) | US Treasury securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | Municipal bonds    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 3) | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 0 0
Recurring basis | (Level 3) | Treasury STRIPS    
Fair value    
Long-term investments 0 0
Short-term investments 0 0
Recurring basis | (Level 3) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments 0 0
Recurring basis | (Level 3) | Money Market    
Fair value    
Nuclear decommissioning trust fund   0
Recurring basis | (Level 3) | Other    
Fair value    
Nuclear decommissioning trust fund 0 0
Long-term investments $ 0 $ 0