Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 80.8	$ 74.5	$ 206.8	$ 200.3
Segment Profit (Loss)	3.1	2.5	15.5	13.4
Capital Expenditures	27.4	28.3	44.9	39.6
Segment Assets	1,137.7	1,051.7	1,137.7	1,051.7	$ 1,128.2
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	32.0	28.9	106.8	102.0
Segment Profit (Loss)	0.1		10.0	9.4
Capital Expenditures	17.0	18.8	22.7	21.9
Segment Assets	637.0	593.8	637.0	593.8	645.2
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	47.4	44.1	96.9	95.2
Segment Profit (Loss)	3.1	2.1	5.2	3.4
Capital Expenditures	5.5	7.3	13.8	13.0
Segment Assets	446.4	419.4	446.4	419.4	441.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	(0.2)	0.1	(0.2)
Capital Expenditures	4.9	2.2	8.4	4.7
Segment Assets	47.2	31.5	47.2	31.5	35.1
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.4	1.5	3.1	3.1
Segment Profit (Loss)	0.1	0.3	0.5	0.6
Segment Assets	$ 7.1	$ 7.0	$ 7.1	$ 7.0	$ 6.8