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Significant Segment Financial Data (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue—Non-Regulated									$ 0.9	$ 4.7
Revenues	$ 116.4	$ 85.3	$ 84.4	$ 152.1	$ 125.6	$ 88.2	$ 84.5	$ 145.8	438.2	444.1	$ 406.2
Interest Income									2.9	2.4	2.4
Interest Expense									26.6	26.4	25.5
Depreciation & Amortization Expense									52.0	50.4	46.9
Income Tax Expense (Benefit)									13.8	8.4	17.5
Segment Profit	11.4	$ 2.3	$ 4.0	$ 26.5	11.0	$ 2.8	$ 3.6	$ 15.6	44.2	33.0	29.0
Segment Assets	1,370.8				1,298.3				1,370.8	1,298.3	1,241.9
Capital Expenditures									119.2	102.4	119.3
Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									435.2	439.4
Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									2.1
Gas Segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									203.4	216.1	194.0
Interest Income									1.2	0.8	0.7
Interest Expense									14.4	14.2	13.7
Depreciation & Amortization Expense									28.5	24.9	22.4
Income Tax Expense (Benefit)									7.2	7.1	10.7
Segment Profit									19.1	18.8	16.4
Segment Assets	823.3				764.1				823.3	764.1	714.3
Capital Expenditures									74.0	70.8	72.1
Gas Segment | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									212.1	210.7
Gas Segment | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									(8.7)	5.4
Electric
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									233.9	223.3	206.2
Interest Income									0.9	0.8	1.0
Interest Expense									9.4	9.0	8.8
Depreciation & Amortization Expense									22.6	23.1	23.4
Income Tax Expense (Benefit)									4.2	4.2	7.5
Segment Profit									11.5	11.4	11.9
Segment Assets	529.3				484.2				529.3	484.2	476.9
Capital Expenditures									39.6	28.4	33.7
Electric | Billed and Unbilled Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									223.1	228.7
Electric | Rate Adjustment Mechanism Revenue
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Revenues									10.8	(5.4)
All Other Segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest Income									0.6	0.6	0.6
Interest Expense									2.8	3.2	3.0
Depreciation & Amortization Expense									0.9	2.3	1.0
Income Tax Expense (Benefit)									(1.4)	(3.4)	(1.4)
Segment Profit									3.4	1.5	(0.5)
Segment Assets	17.9				43.1				17.9	43.1	44.0
Capital Expenditures									5.6	3.2	13.5
Non- Regulated Operation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other Operating Revenue—Non-Regulated									0.9	4.7
Revenues									0.9	4.7	6.0
Interest Income									0.2	0.2	0.1
Depreciation & Amortization Expense										0.1	0.1
Income Tax Expense (Benefit)									3.8	0.5	0.7
Segment Profit									10.2	1.3	1.2
Segment Assets	$ 0.3				$ 6.9				$ 0.3	$ 6.9	$ 6.7

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