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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 101.1 $ 92.9 $ 374.5 $ 367.3
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 90.6 92.8 385.5 369.9
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 10.5 (0.1) (11.0) (2.6)
Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 64.3 62.5 175.5 192.5
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 61.6 67.5 169.4 197.3
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 2.7 (5.0) 6.1 (4.8)
Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 36.8 30.4 199.0 174.8
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 29.0 25.3 216.1 172.6
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 7.8 5.1 (17.1) 2.2
Residential        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 43.7 46.3 177.1 176.7
Residential | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 33.6 37.4 90.2 108.4
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 10.1 8.9 86.9 68.3
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 42.9 42.6 187.8 179.1
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 25.4 27.4 69.9 81.7
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 17.5 15.2 117.9 97.4
Other        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 4.0 3.9 20.6 14.1
Other | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 2.6 2.7 9.3 7.2
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 1.4 $ 1.2 $ 11.3 $ 6.9