Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Jun. 30, 2024 USD ($) mi² segment customer	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 48.4				$ 48.4		$ 41.0
Restricted Cash	0.1				0.1		0.1
Excise taxes collected	11.7		$ 10.6		24.0	$ 22.6
Customer receivables	67.2				67.2		71.0
Unbilled Revenue	20.2				20.2		19.4
Accounts Receivable, Allowance for Credit Loss	(1.0)				(1.0)		(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	1.0		0.7		1.0	0.7	0.9	$ 0.5
Accounts Receivable, Credit Loss Expense (Reversal)					2.3	2.3
Accounts Receivable, Allowance for Credit Loss, Writeoff					(2.7)	(2.6)
Accounts Receivable, Allowance for Credit Loss, Recovery					0.5	0.5
Interest expense	(20.8)		(19.3)		(41.1)	(37.4)
Income Tax Expense (Benefit)	(1.3)		5.4		(2.9)	4.3
Net income	(10.2)	$ (2.6)	5.2	$ (1.2)	(12.8)	4.0
Nonoperating Income (Expense)	18.9		17.8		37.4	34.1
Accumulated other comprehensive income	(5.0)				(5.0)		(4.6)
Other Comprehensive Income (Loss), Net of Tax	(0.2)	(0.2)	(0.1)	(0.2)	(0.4)	(0.3)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.1		0.1		0.1	0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					(0.4)
Contract with Customer, Asset, before Allowance for Credit Loss	87.4				87.4		90.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	48.5		45.2		48.5	45.2	41.1	30.6
Accounts Receivable, after Allowance for Credit Loss, Current	94.4				94.4		92.5
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.3)		(0.2)		(0.5)	(0.4)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	0.1		(0.0)		(0.0)	(0.0)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	(0.0)		0.1		(0.0)	0.1
Allowance for Funds Used During Construction, Equity Portion	2.2		1.1		4.0	2.5
Allowance for Funds Used During Construction, Capitalized Interest	(2.4)		(1.4)		(4.5)	(3.0)
Unrealized Gain (Loss) on Investments	0.0		0.1		0.3	0.4
Nonrelated Party
Significant Accounting Policies [Line Items]
Other	6.0				6.0		2.2
Related Party
Significant Accounting Policies [Line Items]
Other	2.0				2.0		0.8
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	10.7				10.7		11.2
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(15.7)				$ (15.7)		(15.8)
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment					1
Cash and Cash Equivalents, at Carrying Value	22.2				$ 22.2		15.5
Restricted Cash	0.1				$ 0.1		0.1
Approximate number of retail customers | customer					541,000
Service area, square miles | mi²					6,000
Number of Operating Segments | segment					1
Excise taxes collected	11.7		10.6		$ 24.0	22.6
Customer receivables	65.8				65.8		70.0
Unbilled Revenue	20.2				20.2		19.4
Accounts Receivable, Allowance for Credit Loss	(1.0)				(1.0)		(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	1.0		0.7		1.0	0.7	0.9	0.5
Accounts Receivable, Credit Loss Expense (Reversal)					2.3	2.3
Accounts Receivable, Allowance for Credit Loss, Writeoff					2.7	2.6
Interest expense	(11.7)		(9.7)		(22.9)	(18.0)
Income Tax Expense (Benefit)	(3.8)		1.5		(6.1)	0.2
Net income	(15.1)		0.4	(9.6)	(24.5)	(9.2)
Nonoperating Income (Expense)	10.4		8.4		20.4	15.2
Accumulated other comprehensive income	(27.1)				(27.1)		(27.9)
Other Comprehensive Income (Loss), Net of Tax	0.4	$ 0.4	0.1	$ 0.1	0.8	0.2
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.4		0.1		0.8	0.2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					0.8
Contract with Customer, Asset, before Allowance for Credit Loss	86.0				86.0		89.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	22.3		33.5		22.3	33.5	15.6	$ 19.8
Accounts Receivable, after Allowance for Credit Loss, Current	93.7				93.7		93.1
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(0.1)		(0.0)		(0.2)	(0.1)
Allowance for Funds Used During Construction, Equity Portion	2.2		1.1		4.0	2.5
Allowance for Funds Used During Construction, Capitalized Interest	(2.4)		(1.4)		(4.5)	(3.0)
Unrealized Gain (Loss) on Investments	0.0		$ 0.1		0.3	$ 0.4
Subsidiaries [Member] | Nonrelated Party
Significant Accounting Policies [Line Items]
Other	6.1				6.1		2.2
Subsidiaries [Member] | Related Party
Significant Accounting Policies [Line Items]
Other	$ 2.6				$ 2.6		$ 2.4