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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 83.9 $ 84.4 $ 214.3 $ 235.6
Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 86.7 93.1 223.7 255.0
Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (2.8) (8.7) (9.4) (19.4)
Gas Segment        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 33.7 32.6 103.9 119.0
Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 34.5 40.3 110.9 139.5
Gas Segment | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (0.8) (7.7) (7.0) (20.5)
Electric        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 50.2 51.8 110.4 116.6
Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 52.2 52.8 112.8 115.5
Electric | Rate Adjustment Mechanism Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues (2.0) (1.0) (2.4) 1.1
Residential | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 44.5 43.4 110.0 117.9
Residential | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 14.3 15.7 45.4 54.4
Residential | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 30.2 27.7 64.6 63.5
C&I | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 39.5 46.1 106.0 124.8
C&I | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 19.0 23.1 61.3 77.1
C&I | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 20.5 23.0 44.7 47.7
Other | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 2.7 3.6 7.7 12.3
Other | Gas Segment | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues 1.2 1.5 4.2 8.0
Other | Electric | Billed and Unbilled Revenue        
Operating Revenues [Line Items]        
Total Gas and Electric Operating Revenues $ 1.5 $ 2.1 $ 3.5 $ 4.3