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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 110.2 $ 98.1 $ 401.7 $ 333.5
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 105.2 88.1 417.3 343.4
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 5.0 10.0 (15.6) (9.9)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 75.7 65.5 219.2 182.2
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 77.8 65.0 226.7 189.1
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (2.1) 0.5 (7.5) (6.9)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 34.5 32.6 182.5 151.3
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 27.4 23.1 190.6 154.3
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 7.1 9.5 (8.1) (3.0)
Residential | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 51.3 42.9 197.9 164.5
Residential | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 42.5 35.0 125.5 104.0
Residential | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 8.8 7.9 72.4 60.5
Commercial & Industrial | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 48.0 41.2 197.4 164.4
Commercial & Industrial | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 30.2 26.8 86.9 78.2
Commercial & Industrial | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 17.8 14.4 110.5 86.2
Other | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 5.9 4.0 22.0 14.5
Other | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 5.1 3.2 14.3 6.9
Other | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 0.8 $ 0.8 $ 7.7 $ 7.6