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Summary of Significant Accounting Policies - Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 494.8 $ 557.1 $ 563.2
Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 484.6 565.5 553.5
Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 10.2 (8.4) 9.7
Gas Segment      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 246.5 250.6 265.3
Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 235.8 253.8 263.3
Gas Segment | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 10.7 (3.2) 2.0
Electric      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 248.3 306.5 297.9
Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 248.8 311.7 290.2
Electric | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (0.5) (5.2) 7.7
Residential | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 227.6 280.4 258.1
Residential | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 92.6 98.8 98.2
Residential | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 135.0 181.6 159.9
C&I | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 236.9 267.0 266.4
C&I | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 133.5 146.9 153.8
C&I | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 103.4 120.1 112.6
Other | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 20.1 18.1 29.0
Other | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 9.7 8.1 11.3
Other | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 10.4 $ 10.0 $ 17.7