Components of Gas and Electric Operating Revenue (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	$ 83.4	$ 79.4	$ 227.9	$ 203.7
Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	93.8	82.5	246.8	210.4
Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(10.4)	(3.1)	(18.9)	(6.7)
Gas Segment
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	34.7	32.0	121.7	106.8
Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	40.1	36.8	131.5	115.7
Gas Segment | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(5.4)	(4.8)	(9.8)	(8.9)
Electric
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	48.7	47.4	106.2	96.9
Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	53.7	45.7	115.3	94.7
Electric | Rate Adjustment Mechanism Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(5.0)	1.7	(9.1)	2.2
Residential | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	43.3	38.6	113.7	96.9
Residential | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	15.8	15.0	51.7	45.9
Residential | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	27.5	23.6	62.0	51.0
C&I | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	46.9	40.9	121.7	103.0
C&I | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	22.9	20.3	72.8	62.3
C&I | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	24.0	20.6	48.9	40.7
Other | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	5.3	3.0	15.4	10.5
Other | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	2.4	1.5	9.5	7.5
Other | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	2.9	$ 1.5	5.9	$ 3.0
Revenue Reduction - TCJA | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(1.7)		(4.0)
Revenue Reduction - TCJA | Gas Segment | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	(1.0)		(2.5)
Revenue Reduction - TCJA | Electric | Billed and Unbilled Revenue
Operating Revenues [Line Items]
Total Gas and Electric Operating Revenues	$ (0.7)		$ (1.5)