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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 138.8 $ 130.4
Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 159.0 136.9
Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (20.2) (6.5)
Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 78.7 70.2
Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 93.2 76.4
Gas Segment | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (14.5) (6.2)
Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 60.1 60.2
Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 65.8 60.5
Electric | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (5.7) (0.3)
Residential    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 75.8 65.5
Residential | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 37.7 31.1
Residential | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 38.1 34.4
Commercial & Industrial    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 76.0 66.4
Commercial & Industrial | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 50.1 42.3
Commercial & Industrial | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 25.9 24.1
Other    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 7.2 5.0
Other | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 5.4 3.0
Other | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 1.8 $ 2.0