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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 220.2 $ 192.6
Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 219.5 198.4
Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 0.7 (5.8)
Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 108.2 89.2
Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 97.6 84.7
Electric | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 10.6 4.5
Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 112.0 103.4
Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 121.9 113.7
Gas Segment | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (9.9) (10.3)
Residential    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 109.2 95.0
Residential | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 60.1 49.4
Residential | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 49.1 45.6
Commercial & Industrial    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 104.0 95.6
Commercial & Industrial | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 34.8 31.0
Commercial & Industrial | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 69.2 64.6
Other    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 6.3 7.8
Other | Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 2.7 4.3
Other | Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 3.6 $ 3.5