Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	$ 0.0	$ 0.9
Total Operating Revenues	130.4	152.1
Segment Profit (Loss)	15.2	26.5
Identifiable Segment Assets	1,378.4	1,290.2	$ 1,370.8
Capital Expenditures	16.8	10.9
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	136.9	161.9
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(6.5)	(10.7)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	70.2	86.4
Segment Profit (Loss)	12.3	13.7
Identifiable Segment Assets	821.4	771.6
Capital Expenditures	5.8	3.3
Gas Segment \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	76.4	99.2
Gas Segment \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(6.2)	(12.8)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	60.2	64.8
Segment Profit (Loss)	2.6	1.9
Identifiable Segment Assets	537.7	502.3
Capital Expenditures	9.9	6.6
Electric \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	60.5	62.7
Electric \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(0.3)	2.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	0.3	0.8
Identifiable Segment Assets	19.2	16.2
Capital Expenditures	1.1	1.0
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated	0.0	0.9
Total Operating Revenues	0.0	0.9
Segment Profit (Loss)	0.0	10.1
Identifiable Segment Assets	$ 0.1	$ 0.1

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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