Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated		$ 1.1	$ 0.9	$ 3.5
Total Operating Revenues	$ 85.3	88.2	321.8	318.5
Segment Profit (Loss)	2.3	2.8	32.8	22.0
Capital Expenditures	38.0	39.1	76.7	76.3
Segment Assets	1,309.7	1,264.5	1,309.7	1,264.5	$ 1,298.3
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	77.8	80.5	332.8	327.3
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	7.5	6.6	(11.9)	(12.3)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	24.9	25.7	143.9	147.4
Segment Profit (Loss)	(3.9)	(3.2)	10.1	9.1
Capital Expenditures	26.9	30.9	48.3	53.1
Segment Assets	780.7	730.0	780.7	730.0
Gas Segment \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	20.5	19.6	160.0	151.1
Gas Segment \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	4.4	6.1	(16.1)	(3.7)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	60.4	61.4	177.0	167.6
Segment Profit (Loss)	4.4	4.5	9.7	10.2
Capital Expenditures	9.2	7.2	24.1	20.8
Segment Assets	507.6	485.7	507.6	485.7
Electric \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	57.3	60.9	172.8	176.2
Electric \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	3.1	0.5	4.2	(8.6)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	1.8	1.2	2.8	1.8
Capital Expenditures	1.9	1.0	4.3	2.4
Segment Assets	21.1	42.2	21.1	42.2
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated		1.1	0.9	3.5
Total Operating Revenues		1.1	0.9	3.5
Segment Profit (Loss)		0.3	10.2	0.9
Segment Assets	$ 0.3	$ 6.6	$ 0.3	$ 6.6

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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