Segment Information - Significant Segment Financial Data (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	$ 557.1	$ 563.2	$ 473.3
Interest Income	6.2	2.8	1.6
Interest Expense	34.9	28.3	27.2
Depreciation & Amortization Expense	67.4	62.6	59.5
Income Tax Expense (Benefit)	13.2	11.2	11.5
Segment Profit (Loss)	45.2	41.4	36.1
Segment Assets	1,670.4	1,590.4	1,540.3
Capital Expenditures	141.0	122.1	115.0
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	565.5	553.5	466.1
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(8.4)	9.7	7.2
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	306.5	297.9	248.5
Interest Income	2.6	0.9	0.8
Interest Expense	11.2	9.1	9.0
Depreciation & Amortization Expense	26.0	25.4	25.9
Income Tax Expense (Benefit)	4.0	3.1	4.5
Segment Profit (Loss)	17.9	15.7	14.0
Segment Assets	612.6	580.9	584.0
Capital Expenditures	44.2	33.8	38.1
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	311.7	290.2	248.5
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(5.2)	7.7
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	250.6	265.3	224.8
Interest Income	2.4	1.0	0.5
Interest Expense	20.7	16.8	15.3
Depreciation & Amortization Expense	40.4	36.3	32.6
Income Tax Expense (Benefit)	9.4	8.2	7.7
Segment Profit (Loss)	28.8	26.5	23.2
Segment Assets	1,031.8	988.8	935.9
Capital Expenditures	92.7	87.6	75.8
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	253.8	263.3	217.6
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total Operating Revenues	(3.2)	2.0	7.2
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income	1.2	0.9	0.3
Interest Expense	3.0	2.4	2.9
Depreciation & Amortization Expense	1.0	0.9	1.0
Income Tax Expense (Benefit)	(0.2)	(0.1)	(0.7)
Segment Profit (Loss)	(1.5)	(0.8)	(1.1)
Segment Assets	26.0	20.7	20.4
Capital Expenditures	$ 4.1	$ 0.7	$ 1.1