Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 74.7	$ 76.6	$ 324.4	$ 306.0
Segment Profit (Loss)	1.7	1.6	17.0	15.3
Capital Expenditures	32.4	31.6	70.9	60.7
Segment Assets	1,018.0	916.5	1,018.0	916.5	$ 1,000.2
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	21.7	20.9	149.6	139.3
Segment Profit (Loss)	(1.4)	(1.5)	9.5	9.3
Capital Expenditures	22.5	23.1	45.4	39.4
Segment Assets	584.3	505.6	584.3	505.6	566.3
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	51.4	54.2	170.1	162.2
Segment Profit (Loss)	2.5	2.9	6.2	5.2
Capital Expenditures	7.8	7.1	19.5	17.2
Segment Assets	413.8	392.1	413.8	392.1	414.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	0.3		0.4	0.2
Capital Expenditures	2.1	1.3	5.9	3.7
Segment Assets	13.5	12.4	13.5	12.4	13.5
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.6	1.5	4.7	4.5
Segment Profit (Loss)	0.3	0.2	0.9	0.6
Capital Expenditures		0.1	0.1	0.4
Segment Assets	$ 6.4	$ 6.4	$ 6.4	$ 6.4	$ 6.3

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Name Accounting Standards Codification

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