Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 125.8	$ 172.2
Segment Profit	10.9	13.6
Identifiable Segment Assets	1,052.9	1,035.1	$ 1,038.8
Capital Expenditures	11.3	13.0
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	73.1	100.3
Segment Profit	9.4	11.6
Identifiable Segment Assets	596.9	590.4	590.9
Capital Expenditures	3.1	7.2
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	51.1	70.3
Segment Profit	1.3	1.8
Identifiable Segment Assets	420.8	418.6	415.1
Capital Expenditures	5.7	4.4
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit	(0.1)	(0.1)
Identifiable Segment Assets	28.4	20.5	26.3
Capital Expenditures	2.5	1.4
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.6	1.6
Segment Profit	0.3	0.3
Identifiable Segment Assets	$ 6.8	$ 5.6	$ 6.5