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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 557.1 $ 563.2 $ 473.3
Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 565.5 553.5 466.1
Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (8.4) 9.7 7.2
Gas Segment      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 250.6 265.3 224.8
Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 253.8 263.3 217.6
Gas Segment | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (3.2) 2.0 7.2
Electric      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 306.5 297.9 248.5
Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 311.7 290.2 248.5
Electric | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (5.2) 7.7 0.0
Residential | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 280.4 258.1 219.0
Residential | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 98.8 98.2 83.9
Residential | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 181.6 159.9 135.1
C&I | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 267.0 266.4 227.4
C&I | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 146.9 153.8 124.1
C&I | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 120.1 112.6 103.3
Other | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 18.1 29.0 19.7
Other | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 8.1 11.3 9.6
Other | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 10.0 $ 17.7 $ 10.1