v3.25.2
Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 102.6 $ 95.7 $ 273.4 $ 274.4
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 99.8 105.8 294.9 277.1
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 2.8 (10.1) (21.5) (2.7)
Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 51.0 56.4 111.2 130.0
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 47.0 58.8 107.8 129.8
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 4.0 (2.4) 3.4 0.2
Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 51.6 39.3 162.2 144.4
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 52.8 47.0 187.1 147.3
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues (1.2) (7.7) (24.9) (2.9)
Residential        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 44.5 48.8 133.4 130.4
Residential | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 23.4 30.4 56.6 71.0
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 21.1 18.4 76.8 59.4
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 51.0 53.3 144.9 136.5
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 21.0 26.3 44.5 54.3
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 30.0 27.0 100.4 82.2
Other        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 4.3 3.7 16.6 10.2
Other | Electric        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues 2.6 2.1 6.7 4.5
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Electric and Gas Operating Revenues $ 1.7 $ 1.6 $ 9.9 $ 5.7