Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	$ 84.0	$ 78.8	$ 290.8	$ 279.1
Segment Profit (Loss)	2.3	3.5	17.8	16.9
Capital Expenditures	39.3	34.8	84.2	74.4
Segment Assets	1,179.9	1,072.4	1,179.9	1,072.4	$ 1,128.2
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	25.1	22.1	131.9	124.1
Segment Profit (Loss)	(2.1)	(2.2)	7.9	7.2
Capital Expenditures	26.0	22.9	48.7	44.8
Segment Assets	661.2	610.3	661.2	610.3	645.2
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	57.5	55.2	154.4	150.4
Segment Profit (Loss)	4.1	5.3	9.3	8.7
Capital Expenditures	10.0	8.8	23.8	21.8
Segment Assets	461.2	425.4	461.2	425.4	441.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)		0.1	(0.2)	0.1
Capital Expenditures	3.3	3.1	11.7	7.8
Segment Assets	50.2	29.5	50.2	29.5	35.1
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	1.4	1.5	4.5	4.6
Segment Profit (Loss)	0.3	0.3	0.8	0.9
Segment Assets	$ 7.3	$ 7.2	$ 7.3	$ 7.2	$ 6.8

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(12))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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