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INCOME TAXES - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning Balance $ 67.1 $ 65.8 $ 61.8
Gross Increases - Current Year 23.4 11.5 11.3
Gross Increases - Prior Year 0.1    
Gross Decreases - Prior Year (0.1) (2.4) (0.3)
Lapse of Statute of Limitations (9.2) (7.8) (7.0)
Ending Balance 81.3 67.1 65.8
CL&P      
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning Balance 25.5 26.2 25.8
Gross Increases - Current Year 4.0 3.5 3.8
Gross Increases - Prior Year 0.1    
Gross Decreases - Prior Year 0.0 (0.9) (0.6)
Lapse of Statute of Limitations (3.8) (3.3) (2.8)
Ending Balance $ 25.8 $ 25.5 $ 26.2