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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 563.2 $ 473.3 $ 418.6
Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 553.5 466.1 411.9
Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 9.7 7.2 6.7
Gas Segment      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 265.3 224.8 191.4
Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 263.3 217.6 185.2
Gas Segment | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 2.0 7.2 6.2
Electric      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 297.9 248.5 227.2
Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 290.2 248.5 226.7
Electric | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 7.7 0.0 0.5
Residential | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 258.1 219.0 201.8
Residential | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 98.2 83.9 73.1
Residential | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 159.9 135.1 128.7
C&I | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 266.4 227.4 195.9
C&I | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 153.8 124.1 104.5
C&I | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 112.6 103.3 91.4
Other | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 29.0 19.7 14.2
Other | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 11.3 9.6 7.6
Other | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 17.7 $ 10.1 $ 6.6