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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 98.1 $ 87.4 $ 333.5 $ 301.7
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 88.1 80.8 343.4 304.4
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 10.0 6.6 (9.9) (2.7)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 32.6 27.5 151.3 131.4
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 23.1 20.0 154.3 130.9
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 9.5 7.5 (3.0) 0.5
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 65.5 59.9 182.2 170.3
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 65.0 60.8 189.1 173.5
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 0.5 (0.9) (6.9) (3.2)
Residential        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 42.9 42.3 164.5 152.3
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 7.9 7.0 60.5 52.4
Residential | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 35.0 35.3 104.0 99.9
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 41.2 35.7 164.4 141.6
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 14.4 11.6 86.2 72.9
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 26.8 24.1 78.2 68.7
Other        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 4.0 2.8 14.5 10.5
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 0.8 1.4 7.6 5.6
Other | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 3.2 $ 1.4 $ 6.9 $ 4.9