Significant Segment Financial Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	$ 563.2	$ 473.3	$ 418.6
Interest Income	2.8	1.6	2.6
Interest Expense	28.3	27.2	26.4
Depreciation & Amortization Expense	62.6	59.5	54.5
Income Tax Expense (Benefit)	11.2	11.5	10.2
Segment Profit (Loss)	41.4	36.1	32.2
Segment Assets	1,590.4	1,540.3	1,477.9
Capital Expenditures	122.1	115.0	122.6
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	553.5	466.1	411.9
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	9.7	7.2	6.7
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	297.9	248.5	227.2
Interest Income	0.9	0.8	1.1
Interest Expense	9.1	9.0	8.7
Depreciation & Amortization Expense	25.4	25.9	23.8
Income Tax Expense (Benefit)	3.1	4.5	4.7
Segment Profit (Loss)	15.7	14.0	12.9
Segment Assets	580.9	584.0	571.8
Capital Expenditures	33.8	38.1	45.5
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	290.2	248.5	226.7
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	7.7		0.5
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	265.3	224.8	191.4
Interest Income	1.0	0.5	1.1
Interest Expense	16.8	15.3	14.2
Depreciation & Amortization Expense	36.3	32.6	29.8
Income Tax Expense (Benefit)	8.2	7.7	7.3
Segment Profit (Loss)	26.5	23.2	19.3
Segment Assets	988.8	935.9	886.3
Capital Expenditures	87.6	75.8	71.1
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	263.3	217.6	185.2
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	7.2	6.2
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income	0.9	0.3	0.4
Interest Expense	2.4	2.9	3.5
Depreciation & Amortization Expense	0.9	1.0	0.9
Income Tax Expense (Benefit)	(0.1)	(0.7)	(1.8)
Segment Profit (Loss)	(0.8)	(1.1)
Segment Assets	20.7	20.4	19.8
Capital Expenditures	$ 0.7	$ 1.1	$ 6.0