Components of Gas and Electric Operating Revenue (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	$ 87.1	$ 82.6	$ 315.0	$ 286.3
Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	80.5	75.3	327.3	285.6
Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	6.6	7.3	(12.3)	0.7
Gas Segment
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	25.7	25.1	147.4	131.9
Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	19.6	19.8	151.1	135.4
Gas Segment | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	6.1	5.3	(3.7)	(3.5)
Electric
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	61.4	57.5	167.6	154.4
Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	60.9	55.5	176.2	150.2
Electric | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	0.5	2.0	(8.6)	4.2
Residential | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	40.2	36.7	153.9	133.6
Residential | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	6.6	6.8	58.3	52.7
Residential | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	33.6	29.9	95.6	80.9
C&I | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	37.2	35.8	158.8	138.7
C&I | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	11.9	11.5	84.7	73.7
C&I | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	25.3	24.3	74.1	65.0
Other | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	4.1	2.8	19.6	13.3
Other | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	1.5	1.5	11.0	9.0
Other | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	2.6	$ 1.3	8.6	$ 4.3
Revenue Reduction - TCJA | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(1.0)		(5.0)
Revenue Reduction - TCJA | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(0.4)		(2.9)
Revenue Reduction - TCJA | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	$ (0.6)		$ (2.1)