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Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	$ 96.6	$ 83.9	$ 235.4	$ 214.3
Segment Profit (Loss)	2.7	3.1	21.6	18.3
Capital Expenditures	28.4	32.0	42.5	48.8
Segment Assets	1,477.1	1,384.5	1,477.1	1,384.5	$ 1,477.9
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	96.3	86.7	255.3	223.7
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	0.3	(2.8)	(19.9)	(9.4)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	40.0	33.7	118.7	103.9
Segment Profit (Loss)	(0.2)	(0.1)	16.2	12.2
Capital Expenditures	18.9	16.8	23.2	22.6
Segment Assets	878.3	821.1	878.3	821.1
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	38.0	34.5	131.2	110.9
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	2.0	(0.8)	(12.5)	(7.0)
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	56.6	50.2	116.7	110.4
Segment Profit (Loss)	3.4	3.1	6.3	5.7
Capital Expenditures	9.2	14.0	18.7	23.9
Segment Assets	577.3	543.8	577.3	543.8
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	58.3	52.2	124.1	112.8
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(1.7)	(2.0)	(7.4)	(2.4)
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)	(0.6)	0.1	(1.0)	0.4
Capital Expenditures	0.3	1.2	0.6	2.3
Segment Assets	21.5	$ 19.6	21.5	$ 19.6
Non- Regulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	$ 0.1		$ 0.1

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