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Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	$ 98.1	$ 87.4	$ 333.5	$ 301.7
Segment Profit (Loss)	0.0	0.3	21.6	18.6
Capital Expenditures	39.0	44.8	81.5	93.6
Segment Assets	1,509.0	1,423.9	1,509.0	1,423.9	$ 1,477.9
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	88.1	80.8	343.4	304.4
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	10.0	6.6	(9.9)	(2.7)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	32.6	27.5	151.3	131.4
Segment Profit (Loss)	(4.1)	(3.6)	12.1	8.6
Capital Expenditures	28.5	27.4	51.7	50.0
Segment Assets	906.2	845.0	906.2	845.0
Gas Segment \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	23.1	20.0	154.3	130.9
Gas Segment \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	9.5	7.5	(3.0)	0.5
Gas Segment \| Other Operating Revenue Non-Regulated
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	0.0	0.0	0.0	0.0
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	65.5	59.9	182.2	170.3
Segment Profit (Loss)	4.4	4.4	10.7	10.1
Capital Expenditures	10.1	16.0	28.8	39.9
Segment Assets	583.6	556.3	583.6	556.3
Electric \| Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	65.0	60.8	189.1	173.5
Electric \| Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	0.5	(0.9)	(6.9)	(3.2)
Electric \| Other Operating Revenue Non-Regulated
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	0.0	0.0	0.0	0.0
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	(0.3)	(0.5)	(1.3)	(0.1)
Capital Expenditures	0.4	1.4	1.0	3.7
Segment Assets	$ 19.2	$ 22.6	19.2	$ 22.6
Non- Regulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)			$ 0.1