Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	$ 110.2	$ 98.1	$ 401.7	$ 333.5
Segment Profit (Loss)	0.5		26.9	21.6
Identifiable Segment Assets	1,578.9	1,509.0	1,578.9	1,509.0	$ 1,540.3
Capital Expenditures	37.2	39.0	82.5	81.5
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	105.2	88.1	417.3	343.4
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	5.0	10.0	(15.6)	(9.9)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	34.5	32.6	182.5	151.3
Segment Profit (Loss)	(5.3)	(4.1)	14.5	12.1
Identifiable Segment Assets	967.5	906.2	967.5	906.2
Capital Expenditures	28.9	28.5	59.9	51.7
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	27.4	23.1	190.6	154.3
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	7.1	9.5	(8.1)	(3.0)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	75.7	65.5	219.2	182.2
Segment Profit (Loss)	5.9	4.4	13.2	10.7
Identifiable Segment Assets	590.9	583.6	590.9	583.6
Capital Expenditures	8.3	10.1	22.5	28.8
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	77.8	65.0	226.7	189.1
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(2.1)	0.5	(7.5)	(6.9)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	(0.1)	(0.3)	(0.8)	(1.3)
Identifiable Segment Assets	$ 20.5	19.2	20.5	19.2
Capital Expenditures		$ 0.4	$ 0.1	1.0
Non- Regulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)				$ 0.1