v3.26.1
Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Fair value    
Nuclear decommissioning trust funds $ 933,447 $ 855,223
Natural gas swaps    
Fair value    
Derivative asset 15,176 10,642
Domestic equity    
Fair value    
Nuclear decommissioning trust funds 279,655 254,191
Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust funds 119,822 108,799
Long-term investments 30,021 27,967
US Treasury securities    
Fair value    
Nuclear decommissioning trust funds 85,160 85,520
Long-term investments 44,025 36,716
Mortgage backed securities    
Fair value    
Nuclear decommissioning trust funds 62,722 65,814
Long-term investments 24,810 28,996
Domestic mutual funds    
Fair value    
Nuclear decommissioning trust funds 132,450 109,307
Long-term investments 434,624 430,593
Municipal bonds    
Fair value    
Nuclear decommissioning trust funds 3,764 6,627
Federal agency securities    
Fair value    
Nuclear decommissioning trust funds 8,093 9,082
Treasury STRIPS    
Fair value    
Long-term investments 53,584 76,392
Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust funds 19,756 14,234
Long-term investments 3,039 3,294
International mutual funds    
Fair value    
Nuclear decommissioning trust funds 206,888 180,112
Long-term investments 55,829 48,935
Other    
Fair value    
Nuclear decommissioning trust funds 15,137 21,537
Long-term investments 412 372
Short-term investments: Treasury STRIPS    
Fair value    
Long-term investments 66,161 80,164
(Level 1) | Natural gas swaps    
Fair value    
Derivative asset 0 0
(Level 1) | Domestic equity    
Fair value    
Nuclear decommissioning trust funds 279,655 254,191
(Level 1) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 1) | US Treasury securities    
Fair value    
Nuclear decommissioning trust funds 85,160 85,520
Long-term investments 44,025 36,716
(Level 1) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 1) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust funds 132,450 109,307
Long-term investments 434,624 430,593
(Level 1) | Municipal bonds    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 1) | Federal agency securities    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 1) | Treasury STRIPS    
Fair value    
Long-term investments 0 0
(Level 1) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 1) | International mutual funds    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 1) | Other    
Fair value    
Nuclear decommissioning trust funds 15,137 21,537
Long-term investments 412 372
(Level 1) | Short-term investments: Treasury STRIPS    
Fair value    
Long-term investments 0 0
(Level 2) | Natural gas swaps    
Fair value    
Derivative asset 15,176 10,642
(Level 2) | Domestic equity    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 2) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust funds 119,642 107,808
Long-term investments 30,021 27,967
(Level 2) | US Treasury securities    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 2) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust funds 62,722 65,814
Long-term investments 24,810 28,996
(Level 2) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 2) | Municipal bonds    
Fair value    
Nuclear decommissioning trust funds 3,764 6,627
(Level 2) | Federal agency securities    
Fair value    
Nuclear decommissioning trust funds 8,093 9,082
(Level 2) | Treasury STRIPS    
Fair value    
Long-term investments 53,584 76,392
(Level 2) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust funds 19,756 14,234
Long-term investments 3,039 3,294
(Level 2) | International mutual funds    
Fair value    
Nuclear decommissioning trust funds 206,888 180,112
Long-term investments 55,829 48,935
(Level 2) | Other    
Fair value    
Nuclear decommissioning trust funds 0
Long-term investments 0 0
(Level 2) | Short-term investments: Treasury STRIPS    
Fair value    
Long-term investments 66,161 80,164
(Level 3) | Natural gas swaps    
Fair value    
Derivative asset 0 0
(Level 3) | Domestic equity    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 3) | Corporate bonds and debt    
Fair value    
Nuclear decommissioning trust funds 180 991
Long-term investments 0 0
(Level 3) | US Treasury securities    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 3) | Mortgage backed securities    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 3) | Domestic mutual funds    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 3) | Municipal bonds    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 3) | Federal agency securities    
Fair value    
Nuclear decommissioning trust funds 0 0
(Level 3) | Treasury STRIPS    
Fair value    
Long-term investments 0 0
(Level 3) | Non-US Gov't bonds & private placements    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 3) | International mutual funds    
Fair value    
Nuclear decommissioning trust funds 0 0
Long-term investments 0 0
(Level 3) | Other    
Fair value    
Nuclear decommissioning trust funds 0
Long-term investments 0 0
(Level 3) | Short-term investments: Treasury STRIPS    
Fair value    
Long-term investments $ 0 $ 0