v3.26.1
Summary of Significant Accounting Policies - Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 117.0 $ 102.6 $ 333.9 $ 273.4
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 125.5 99.8 352.5 294.9
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 8.5 2.8 (18.6) (21.5)
Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 61.7 51.0 127.2 111.2
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 60.5 47.0 132.2 107.8
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 1.2 4.0 5.0 3.4
Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 55.3 51.6 206.7 162.2
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 65.0 52.8 220.3 187.1
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues (9.7) (1.2) (13.6) (24.9)
Residential        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 52.8 44.5 155.0 133.4
Residential | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 27.8 23.4 66.1 56.6
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 25.0 21.1 88.9 76.8
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 63.6 51.0 178.2 144.9
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 25.8 21.0 53.0 44.5
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 37.8 30.0 125.2 100.4
Other        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 9.1 4.3 19.3 16.6
Other | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 6.9 2.6 13.1 6.7
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 2.2 $ 1.7 $ 6.2 $ 9.9