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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 437.3 $ 439.4 $ 400.2
Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 435.2 439.4 389.1
Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 2.1   11.1
Gas Segment      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 203.4 216.1 194.0
Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 212.1 210.7 187.5
Gas Segment | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (8.7) 5.4 6.5
Electric      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 233.9 223.3 206.2
Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 223.1 228.7 201.6
Electric | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 10.8 (5.4) 4.6
Residential | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 202.9 205.0 179.1
Residential | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 81.4 81.4 71.2
Residential | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 121.5 123.6 107.9
Commercial & Industrial | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 213.9 216.1 190.5
Commercial & Industrial | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 120.1 119.7 102.8
Commercial & Industrial | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 93.8 96.4 87.7
Other | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 18.4 18.3 19.5
Other | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 10.6 9.6 13.5
Other | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 7.8 $ 8.7 $ 6.0