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Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 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	$ 0.0		$ 0.9
Total Operating Revenues	83.9	84.4	$ 214.3	236.5
Segment Profit (Loss)	3.1	4.0	18.3	30.5
Capital Expenditures	32.0	27.8	48.8	38.7
Segment Assets	1,384.5	1,279.3	1,384.5	1,279.3	$ 1,370.8
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	86.7	93.1	223.7	255.0
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(2.8)	(8.7)	(9.4)	(19.4)
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	33.7	32.6	103.9	119.0
Segment Profit (Loss)	(0.1)	0.3	12.2	14.0
Capital Expenditures	16.8	18.1	22.6	21.4
Segment Assets	821.1	760.0	821.1	760.0
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	34.5	40.3	110.9	139.5
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(0.8)	(7.7)	(7.0)	(20.5)
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	50.2	51.8	110.4	116.6
Segment Profit (Loss)	3.1	3.4	5.7	5.3
Capital Expenditures	14.0	8.3	23.9	14.9
Segment Assets	543.8	500.4	543.8	500.4
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	52.2	52.8	112.8	115.5
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(2.0)	(1.0)	(2.4)	1.1
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment Profit (Loss)	0.1	0.2	0.4	1.0
Capital Expenditures	1.2	1.4	2.3	2.4
Segment Assets	$ 19.6	18.3	$ 19.6	18.3
Unregulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue - Non-Regulated				0.9
Total Operating Revenues				0.9
Segment Profit (Loss)		0.1		10.2
Segment Assets		$ 0.6		$ 0.6

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