Significant Segment Financial Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue – Non-Regulated			$ 0.9
Total Operating Revenues	$ 473.3	$ 418.6	438.2
Interest Income	1.6	2.6	2.9
Interest Expense	27.2	26.4	26.6
Depreciation & Amortization Expense	59.5	54.5	52.0
Income Tax Expense (Benefit)	11.5	10.2	13.8
Segment Profit (Loss)	36.1	32.2	44.2
Segment Assets	1,540.3	1,477.9	1,370.8
Capital Expenditures	115.0	122.6	119.2
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	466.1	411.9	435.2
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	7.2	6.7	2.1
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	248.5	227.2	233.9
Interest Income	0.8	1.1	0.9
Interest Expense	9.0	8.7	9.4
Depreciation & Amortization Expense	25.9	23.8	22.6
Income Tax Expense (Benefit)	4.5	4.7	4.2
Segment Profit (Loss)	14.0	12.9	11.5
Segment Assets	584.0	571.8	529.3
Capital Expenditures	38.1	45.5	39.6
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	248.5	226.7	223.1
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	0.0	0.5	10.8
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	224.8	191.4	203.4
Interest Income	0.5	1.1	1.2
Interest Expense	15.3	14.2	14.4
Depreciation & Amortization Expense	32.6	29.8	28.5
Income Tax Expense (Benefit)	7.7	7.3	7.2
Segment Profit (Loss)	23.2	19.3	19.1
Segment Assets	935.9	886.3	823.3
Capital Expenditures	75.8	71.1	74.0
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	217.6	185.2	212.1
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	7.2	6.2
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income	0.3	0.4	0.6
Interest Expense	2.9	3.5	2.8
Depreciation & Amortization Expense	1.0	0.9	0.9
Income Tax Expense (Benefit)	(0.6)	(1.8)	(1.4)
Segment Profit (Loss)	(1.2)		3.4
Segment Assets	20.4	19.8	17.9
Capital Expenditures	1.1	$ 6.0	5.6
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
Interest Income			0.2
Income Tax Expense (Benefit)	(0.1)		3.8
Segment Profit (Loss)	$ 0.1		10.2
Segment Assets			$ 0.3