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Summary of Significant Accounting Policies - Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 92.9 $ 103.9 $ 367.3 $ 427.5
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 92.8 105.0 369.9 439.8
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 0.1 (1.1) (2.6) (12.3)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 62.5 72.1 192.5 244.8
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 67.5 80.6 197.3 248.1
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (5.0) (8.5) (4.8) (3.3)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 30.4 31.8 174.8 182.7
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 25.3 24.4 172.6 191.7
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 5.1 7.4 2.2 (9.0)
Residential        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 46.3 55.9 176.7 222.9
Residential | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 37.4 47.5 108.4 147.5
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 8.9 8.4 68.3 75.4
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 42.6 45.6 179.1 204.2
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 27.4 30.8 81.7 93.7
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 15.2 14.8 97.4 110.5
Other        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 3.9 3.5 14.1 12.7
Other | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 2.7 2.3 7.2 6.9
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 1.2 $ 1.2 $ 6.9 $ 5.8