Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	$ 1.1		$ 3.5
Revenues	88.2	$ 84.0	318.5	$ 290.8
Segment Profit (Loss)	2.8	2.3	22.0	17.8
Capital Expenditures	39.1	39.3	76.3	84.2
Segment Assets	1,264.5	1,179.9	1,264.5	1,179.9	$ 1,241.9
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	80.5		327.3
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	6.6		(12.3)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	25.7	25.1	147.4	131.9
Segment Profit (Loss)	(3.2)	(2.1)	9.1	7.9
Capital Expenditures	30.9	26.0	53.1	48.7
Segment Assets	730.0	661.2	730.0	661.2	714.3
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	19.6		151.1
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	6.1		(3.7)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	61.4	57.5	167.6	154.4
Segment Profit (Loss)	4.5	4.1	10.2	9.3
Capital Expenditures	7.2	10.0	20.8	23.8
Segment Assets	485.7	461.2	485.7	461.2	476.9
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	60.9		176.2
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	0.5		(8.6)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	1.2		1.8	(0.2)
Capital Expenditures	1.0	3.3	2.4	11.7
Segment Assets	42.2	50.2	42.2	50.2	44.0
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	1.1		3.5
Revenues	1.1	1.4	3.5	4.5
Segment Profit (Loss)	0.3	0.3	0.9	0.8
Segment Assets	$ 6.6	$ 7.3	$ 6.6	$ 7.3	$ 6.7