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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 103.4 $ 98.9 $ 323.6 $ 291.5
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 115.1 113.7 334.6 312.1
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (11.7) (14.8) (11.0) (20.6)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 64.5 54.3 172.7 143.5
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 69.8 64.1 167.4 148.8
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (5.3) (9.8) 5.3 (5.3)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 38.9 44.6 150.9 148.0
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 45.3 49.6 167.2 163.3
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (6.4) (5.0) (16.3) (15.3)
Residential        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 57.7 51.9 166.9 146.6
Residential | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 39.8 33.8 99.9 83.0
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 17.9 18.1 67.0 63.6
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 54.5 53.7 158.5 149.4
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 28.1 25.7 62.9 56.7
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 26.4 28.0 95.6 92.7
Other        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 2.9 8.1 9.2 16.1
Other | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 1.9 4.6 4.6 9.1
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 1.0 $ 3.5 $ 4.6 $ 7.0