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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 103.9 $ 110.2 $ 427.5 $ 401.7
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 105.0 105.2 439.8 417.3
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (1.1) (5.0) (12.3) (15.6)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 72.1 75.7 244.8 219.2
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 80.6 77.8 248.1 226.7
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (8.5) (2.1) 3.3 (7.5)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 31.8 34.5 182.7 182.5
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 24.4 27.4 191.7 190.6
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 7.4 (7.1) (9.0) (8.1)
Residential        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 55.9 51.3 222.9 197.9
Residential | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 47.5 42.5 147.5 125.5
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 8.4 8.8 75.4 72.4
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 45.6 48.0 204.2 197.4
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 30.8 30.2 93.7 86.9
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 14.8 17.8 110.5 110.5
Other        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 3.5 5.9 12.7 22.0
Other | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 2.3 5.1 6.9 14.3
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 1.2 $ 0.8 $ 5.8 $ 7.7