v3.25.1
Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on A Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2025
Dec. 31, 2024
Fair value    
Nuclear decommissioning trust fund $ 723,015 $ 721,624
Long-term investments 638,561 645,166
Short-term investments 119,860 124,572
Recurring basis | Natural gas swaps    
Fair value    
Derivative liabilities 59,792 28,624
Recurring basis | Domestic equity    
Fair value    
Nuclear decommissioning trust fund 234,410 245,313
Recurring basis | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust fund 87,115 82,316
Long-term investments 22,432 20,972
Recurring basis | US Treasury securities    
Fair value    
Nuclear decommissioning trust fund 55,750 53,806
Long-term investments 26,358 25,654
Recurring basis | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust fund 71,984 70,193
Long-term investments 21,551 20,232
Recurring basis | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust fund 89,061 95,175
Long-term investments 387,612 394,595
Recurring basis | Municipal bonds    
Fair value    
Nuclear decommissioning trust fund 8,099 3,375
Recurring basis | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 9,243 8,487
Recurring basis | Treasury STRIPS    
Fair value    
Long-term investments 134,822 142,199
Short-term investments 119,860 124,572
Recurring basis | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 4,545 3,319
Long-term investments 2,744 1,805
Recurring basis | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 153,135 4,228
Long-term investments 42,439 39,340
Recurring basis | Money Market    
Fair value    
Nuclear decommissioning trust fund   138,553
Recurring basis | Other    
Fair value    
Nuclear decommissioning trust fund 9,673 16,859
Long-term investments 603 369
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 234,410 245,313
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 55,750 53,806
Long-term investments 26,358 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 89,061 95,175
Long-term investments 387,612 394,595
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 9,673 16,859
Long-term investments 603 369
Recurring basis | (Level 2) | Natural gas swaps    
Fair value    
Derivative liabilities 59,792 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 86,529 82,309
Long-term investments 22,432 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 71,984 70,193
Long-term investments 21,551 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 8,099 3,375
Recurring basis | (Level 2) | Federal agency securities    
Fair value    
Nuclear decommissioning trust fund 9,243 8,487
Recurring basis | (Level 2) | Treasury STRIPS    
Fair value    
Long-term investments 134,822 142,199
Short-term investments 119,860 124,572
Recurring basis | (Level 2) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust fund 4,545 3,319
Long-term investments 2,744 1,805
Recurring basis | (Level 2) | International mutual funds    
Fair value    
Nuclear decommissioning trust fund 153,135 4,228
Long-term investments 42,439 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 586 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