v3.26.1
Summary of Significant Accounting Policies - Regulatory Assets (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Jun. 30, 2025
Regulatory Assets [Line Items]      
Regulatory Assets $ 103.9 $ 116.9 $ 99.2
Less: Current Portion of Regulatory Assets [1] 64.8 78.1 52.6
Regulatory Assets – noncurrent 39.1 38.8 46.6
Other Deferred Charges      
Regulatory Assets [Line Items]      
Regulatory Assets 16.7 12.8 14.7
Environmental Matters      
Regulatory Assets [Line Items]      
Regulatory Assets 10.5 10.0 9.4
Retirement Benefits      
Regulatory Assets [Line Items]      
Regulatory Assets 13.7 14.6 19.3
Deferred Storm Charges      
Regulatory Assets [Line Items]      
Regulatory Assets 4.5 6.0 7.0
Income Taxes      
Regulatory Assets [Line Items]      
Regulatory Assets 0.2 0.4 0.7
Energy Supply and Other Rate Adjustment Mechanisms      
Regulatory Assets [Line Items]      
Regulatory Assets $ 58.3 $ 73.1 $ 48.1
[1] Reflects amounts included in the Accrued Revenue on the Company’s Consolidated Balance Sheets.