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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues $ 170.8 $ 178.7
Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 195.1 171.4
Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues (24.3) 7.3
Electric    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 60.2 73.6
Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 60.8 71.0
Electric | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues (0.6) 2.6
Gas Segment    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 110.6 105.1
Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 134.3 100.4
Gas Segment | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues (23.7) 4.7
Residential    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 89.0 81.6
Residential | Electric    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 33.2 40.5
Residential | Gas Segment    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 55.8 41.1
Commercial & Industrial    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 93.7 83.3
Commercial & Industrial | Electric    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 23.4 28.1
Commercial & Industrial | Gas Segment    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 70.3 55.2
Other    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 12.4 6.5
Other | Electric    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues 4.2 2.4
Other | Gas Segment    
Operating Revenues [Line Items]    
Total Electric and Gas Operating Revenues $ 8.2 $ 4.1