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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 130.4 $ 151.2
Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 136.9 161.9
Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (6.5) (10.7)
Gas Segment    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 70.2 86.4
Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 76.4 99.2
Gas Segment | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (6.2) (12.8)
Electric    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 60.2 64.8
Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 60.5 62.7
Electric | Rate Adjustment Mechanism Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues (0.3) 2.1
Residential | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 65.5 74.4
Residential | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 31.1 38.7
Residential | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 34.4 35.7
C&I | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 66.4 78.7
C&I | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 42.3 54.0
C&I | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 24.1 24.7
Other | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 5.0 8.8
Other | Gas Segment | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues 3.0 6.5
Other | Electric | Billed and Unbilled Revenue    
Operating Revenues [Line Items]    
Total Gas and Electric Operating Revenues $ 2.0 $ 2.3