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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 96.6 $ 83.9 $ 235.4 $ 214.3
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 96.3 86.7 255.3 223.7
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 0.3 (2.8) (19.9) (9.4)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 40.0 33.7 118.7 103.9
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 38.0 34.5 131.2 110.9
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 2.0 (0.8) (12.5) (7.0)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 56.6 50.2 116.7 110.4
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 58.3 52.2 124.1 112.8
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (1.7) (2.0) (7.4) (2.4)
Residential        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 45.7 44.5 121.5 110.0
Residential | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 14.9 14.3 52.6 45.4
Residential | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 30.8 30.2 68.9 64.6
Commercial & Industrial        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 47.3 39.5 123.3 106.0
Commercial & Industrial | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 21.8 19.0 71.9 61.3
Commercial & Industrial | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 25.5 20.5 51.4 44.7
Other        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 3.3 2.7 10.5 7.7
Other | Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 1.3 1.2 6.7 4.2
Other | Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 2.0 $ 1.5 $ 3.8 $ 3.5