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Components of Gas and Electric Operating Revenue (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ 439.4 $ 400.2 $ 377.3
Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 439.4 389.1 353.7
Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues   11.1 23.6
Gas Segment      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 216.1 194.0 181.2
Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 210.7 187.5 165.4
Gas Segment | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 5.4 6.5 15.8
Electric      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 223.3 206.2 196.1
Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 228.7 201.6 188.3
Electric | Rate Adjustment Mechanism Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (5.4) 4.6 7.8
Residential | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 205.0 179.1 163.4
Residential | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 81.4 71.2 61.5
Residential | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 123.6 107.9 101.9
C&I | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 216.1 190.5 174.2
C&I | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 119.7 102.8 92.7
C&I | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 96.4 87.7 81.5
Other | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 24.6 19.5 16.1
Other | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 13.3 13.5 11.2
Other | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues 11.3 $ 6.0 $ 4.9
Revenue Reduction - TCJA | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (6.3)    
Revenue Reduction - TCJA | Gas Segment | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues (3.7)    
Revenue Reduction - TCJA | Electric | Billed and Unbilled Revenue      
Operating Revenues [Line Items]      
Total Gas and Electric Operating Revenues $ (2.6)