Significant Segment Financial Data (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 66.4	$ 68.8	$ 184.6	$ 183.0
Segment Profit (Loss)	(0.1)	(0.4)	10.7	8.6
Capital Expenditures	22.6	14.8	37.0	23.9
Segment Assets	874.2	824.7	874.2	824.7	892.3
Electricity
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	42.9	44.8	88.8	93.5
Segment Profit (Loss)	1.4	1.4	3.4	2.6
Capital Expenditures	3.4	4.1	10.4	9.4
Segment Assets	405.7	398.3	405.7	398.3
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	22.1	22.7	92.9	86.9
Segment Profit (Loss)	(2.3)	(2.2)	6.0	5.4
Capital Expenditures	17.6	10.0	24.5	13.1
Segment Assets	456.8	416.1	456.8	416.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	0.6	0.1	0.7
Capital Expenditures	1.6	0.7	2.1	1.4
Segment Assets	5.3	3.3	5.3	3.3
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.4	1.3	2.9	2.6
Segment Profit (Loss)	0.2	0.3	0.6	0.6
Segment Assets	$ 6.4	$ 7.0	$ 6.4	$ 7.0