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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 192.6 $ 138.8
Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 198.4 159.0
Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (5.8) (20.2)
Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 89.2 60.1
Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 84.7 65.8
Electric | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 4.5 (5.7)
Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 103.4 78.7
Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 113.7 93.2
Gas Segment | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (10.3) (14.5)
Residential | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 95.0 75.8
Residential | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 49.4 38.1
Residential | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 45.6 37.7
Commercial & Industrial | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 95.6 76.0
Commercial & Industrial | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 31.0 25.9
Commercial & Industrial | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 64.6 50.1
Other | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 7.8 7.2
Other | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 4.3 1.8
Other | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 3.5 $ 5.4