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Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
ASSETS    
Cash equivalents $ 15  
Commodity contracts, assets   $ 132,229
Commodity contracts assets, other   (21,163)
Nuclear decommissioning trust: 1,120,463 1,073,410
Nuclear decommissioning trust, other 396,716 476,409
Other special use funds 368,159 347,231
Other special use funds, other 1,282 963
Total assets 1,503,184 1,552,870
Total assets, other 384,097 456,209
LIABILITIES    
Gross derivative liability, other   15,357
Interest rate swaps   (42,446)
Total liabilities (55,744) (42,446)
Total liabilities, other 6,835 15,357
Commodity Option    
ASSETS    
Commodity contracts, assets 14,547 132,098
Commodity contracts assets, other (13,901) (21,163)
LIABILITIES    
Gross derivative liability, other 6,835 15,357
Interest rate swaps (55,744) (41,537)
Interest Rate Swap    
ASSETS    
Commodity contracts, assets   131
Commodity contracts assets, other   0
LIABILITIES    
Gross derivative liability, other   0
Interest rate swaps   (909)
Equity securities    
ASSETS    
Nuclear decommissioning trust: 13,127 18,485
Nuclear decommissioning trust, other (17,110) 3,827
Other special use funds 56,535 67,937
Other special use funds, other 1,282 963
U.S. commingled equity funds    
ASSETS    
Nuclear decommissioning trust: 413,826 472,582
U.S. Treasury debt    
ASSETS    
Nuclear decommissioning trust: 273,702 211,923
Other special use funds 307,710 275,267
Corporate debt    
ASSETS    
Nuclear decommissioning trust: 173,383 149,226
Mortgage-backed securities    
ASSETS    
Nuclear decommissioning trust: 181,335 147,938
Municipal bonds    
ASSETS    
Nuclear decommissioning trust: 59,553 64,881
Other special use funds 3,914 4,027
Other fixed income    
ASSETS    
Nuclear decommissioning trust: 5,537 8,375
Level 1    
ASSETS    
Cash equivalents 15  
Commodity contracts, assets   0
Nuclear decommissioning trust: 303,939 226,581
Other special use funds 362,963 342,241
Total assets 666,917 568,822
LIABILITIES    
Gross derivative liability   0
Total liabilities 0 0
Level 1 | Commodity Option    
ASSETS    
Commodity contracts, assets 0 0
LIABILITIES    
Gross derivative liability 0 0
Level 1 | Interest Rate Swap    
ASSETS    
Commodity contracts, assets   0
LIABILITIES    
Gross derivative liability   0
Level 1 | Equity securities    
ASSETS    
Nuclear decommissioning trust: 30,237 14,658
Other special use funds 55,253 66,974
Level 1 | U.S. commingled equity funds    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 1 | U.S. Treasury debt    
ASSETS    
Nuclear decommissioning trust: 273,702 211,923
Other special use funds 307,710 275,267
Level 1 | Corporate debt    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 1 | Mortgage-backed securities    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 1 | Municipal bonds    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Level 1 | Other fixed income    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 2    
ASSETS    
Cash equivalents 0  
Commodity contracts, assets   127,260
Nuclear decommissioning trust: 419,808 370,420
Other special use funds 3,914 4,027
Total assets 445,345 501,707
LIABILITIES    
Gross derivative liability   (26,783)
Total liabilities (62,579) (26,783)
Level 2 | Commodity Option    
ASSETS    
Commodity contracts, assets 21,623 127,129
LIABILITIES    
Gross derivative liability (62,579) (25,874)
Level 2 | Interest Rate Swap    
ASSETS    
Commodity contracts, assets   131
LIABILITIES    
Gross derivative liability   (909)
Level 2 | Equity securities    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Level 2 | U.S. commingled equity funds    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 2 | U.S. Treasury debt    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Level 2 | Corporate debt    
ASSETS    
Nuclear decommissioning trust: 173,383 149,226
Level 2 | Mortgage-backed securities    
ASSETS    
Nuclear decommissioning trust: 181,335 147,938
Level 2 | Municipal bonds    
ASSETS    
Nuclear decommissioning trust: 59,553 64,881
Other special use funds 3,914 4,027
Level 2 | Other fixed income    
ASSETS    
Nuclear decommissioning trust: 5,537 8,375
Level 3    
ASSETS    
Cash equivalents 0  
Commodity contracts, assets   26,132
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Total assets 6,825 26,132
LIABILITIES    
Gross derivative liability   (31,020)
Total liabilities 0 (31,020)
Level 3 | Commodity Option    
ASSETS    
Commodity contracts, assets 6,825 26,132
LIABILITIES    
Gross derivative liability 0 (31,020)
Level 3 | Interest Rate Swap    
ASSETS    
Commodity contracts, assets   0
LIABILITIES    
Gross derivative liability   0
Level 3 | Equity securities    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Level 3 | U.S. commingled equity funds    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 3 | U.S. Treasury debt    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
Level 3 | Corporate debt    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 3 | Mortgage-backed securities    
ASSETS    
Nuclear decommissioning trust: 0 0
Level 3 | Municipal bonds    
ASSETS    
Nuclear decommissioning trust: 0 0
Other special use funds 0 0
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: $ 413,826 $ 472,582