Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 77.5	$ 73.3	$ 249.7	$ 229.4
Segment Profit (Loss)	1.7	1.1	15.3	13.7
Capital Expenditures	25.4	19.9	38.5	29.1
Segment Assets	1,011.4	900.6	1,011.4	900.6	$ 1,000.2
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	27.6	25.8	127.9	118.4
Segment Profit (Loss)	(0.7)	(0.7)	10.9	10.8
Capital Expenditures	15.7	13.3	22.9	16.3
Segment Assets	587.4	490.6	587.4	490.6	566.3
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	48.4	46.1	118.7	108.0
Segment Profit (Loss)	1.9	1.4	3.7	2.3
Capital Expenditures	7.3	5.0	11.7	10.1
Segment Assets	410.9	391.7	410.9	391.7	414.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	0.2	0.2	0.1	0.2
Capital Expenditures	2.3	1.4	3.8	2.4
Segment Assets	7.1	12.3	7.1	12.3	13.5
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.5	1.4	3.1	3.0
Segment Profit (Loss)	0.3	0.2	0.6	0.4
Capital Expenditures	0.1	0.2	0.1	0.3
Segment Assets	$ 6.0	$ 6.0	$ 6.0	$ 6.0	$ 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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