Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	$ 1.1		$ 2.4
Revenues	84.5	$ 80.8	230.3	$ 206.8
Segment Profit (Loss)	3.6	3.1	19.2	15.5
Capital Expenditures	27.1	27.4	37.2	44.9
Segment Assets	1,234.6	1,137.7	1,234.6	1,137.7	$ 1,241.9
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	93.8		246.8
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	(10.4)		(18.9)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	34.7	32.0	121.7	106.8
Segment Profit (Loss)	(0.3)	0.1	12.3	10.0
Capital Expenditures	18.6	17.0	22.2	22.7
Segment Assets	703.2	637.0	703.2	637.0	714.3
Gas Segment \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	40.1		131.5
Gas Segment \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	(5.4)		(9.8)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	48.7	47.4	106.2	96.9
Segment Profit (Loss)	2.7	3.1	5.7	5.2
Capital Expenditures	7.6	5.5	13.6	13.8
Segment Assets	479.3	446.4	479.3	446.4	476.9
Electric \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	53.7		115.3
Electric \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues	(5.0)		(9.1)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	1.0	(0.2)	0.6	(0.2)
Capital Expenditures	0.9	4.9	1.4	8.4
Segment Assets	45.7	47.2	45.7	47.2	44.0
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	1.1		2.4
Revenues	1.1	1.4	2.4	3.1
Segment Profit (Loss)	0.2	0.1	0.6	0.5
Segment Assets	$ 6.4	$ 7.1	$ 6.4	$ 7.1	$ 6.7

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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