Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024 USD ($) mi² customer segment	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 24.2		$ 41.0
Restricted Cash	0.1		0.1
Excise taxes collected	12.3	$ 12.0
Customer receivables	76.7		71.0
Unbilled Revenue	16.5		19.4
Accounts Receivable, Allowance for Credit Loss	(0.9)		(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	0.9	0.6	0.9	$ 0.5
Accounts Receivable, Credit Loss Expense (Reversal)	0.8	0.9
Accounts Receivable, Allowance for Credit Loss, Writeoff	(1.0)	(1.1)
Accounts Receivable, Allowance for Credit Loss, Recovery	0.2	0.3
Interest expense	(20.3)	(18.1)
Income Tax Expense (Benefit)	(1.6)	(1.1)
Net income	(2.6)	(1.2)
Nonoperating Income (Expense)	18.5	16.3
Accumulated other comprehensive income	(4.8)		(4.6)
Other Comprehensive Income (Loss), Net of Tax	(0.2)	(0.2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.2)	(0.2)
Contract with Customer, Asset, before Allowance for Credit Loss	93.2		90.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	24.3	21.1	41.1	30.6
Accounts Receivable, after Allowance for Credit Loss, Current	99.8		92.5
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.2)	(0.2)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(0.1)	0.0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	0.0	0.0
Allowance for Funds Used During Construction, Equity Portion	1.8	1.4
Allowance for Funds Used During Construction, Capitalized Interest	(2.1)	(1.6)
Unrealized Gain (Loss) on Investments	0.3	0.3
Nonrelated Party
Significant Accounting Policies [Line Items]
Other	5.9		2.2
Related Party
Significant Accounting Policies [Line Items]
Other	1.6		0.8
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	11.0		11.2
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(15.8)		(15.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Interest expense	(0.2)	(0.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Nonoperating Income (Expense)	$ 0.1	0.0
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Cash and Cash Equivalents, at Carrying Value	$ 7.6		15.5
Restricted Cash	$ 0.1		0.1
Approximate number of retail customers | customer	540,000
Service area, square miles | mi²	6,000
Number of Operating Segments | segment	1
Excise taxes collected	$ 12.3	12.0
Customer receivables	75.6		70.0
Unbilled Revenue	16.5		19.4
Accounts Receivable, Allowance for Credit Loss	(0.9)		(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	0.9	0.6	0.9	0.5
Accounts Receivable, Credit Loss Expense (Reversal)	0.8	0.9
Accounts Receivable, Allowance for Credit Loss, Writeoff	1.0	1.1
Interest expense	(11.2)	(8.3)
Income Tax Expense (Benefit)	(2.3)	(1.3)
Net income	(9.4)	(9.6)
Nonoperating Income (Expense)	10.0	6.8
Accumulated other comprehensive income	(27.5)		(27.9)
Other Comprehensive Income (Loss), Net of Tax	0.4	0.1
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.4	0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.4	0.1
Contract with Customer, Asset, before Allowance for Credit Loss	92.1		89.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7.7	9.5	15.6	$ 19.8
Accounts Receivable, after Allowance for Credit Loss, Current	100.7		93.1
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax		(0.1)	(0.1)
Allowance for Funds Used During Construction, Equity Portion	1.8	1.4
Allowance for Funds Used During Construction, Capitalized Interest	(2.1)	(1.6)
Unrealized Gain (Loss) on Investments	0.3	0.3
Subsidiaries [Member] | Nonrelated Party
Significant Accounting Policies [Line Items]
Other	5.8		2.2
Subsidiaries [Member] | Related Party
Significant Accounting Policies [Line Items]
Other	3.7		$ 2.4
Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Net income	0.4	0.1
Nonoperating Income (Expense)	$ 0.5	$ 0.2