Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Common Stock, Shares Authorized	1,500	1,500
Common stock, shares issued	1	1
Accounts Receivable, Allowance for Credit Loss, Current	$ 0.9	$ 0.9
Cash and cash equivalents	24.2	41.0
Accounts Receivable, after Allowance for Credit Loss, Current	99.8	92.5
Inventories	52.0	44.5
Taxes applicable to subsequent years	83.4	113.0
Regulatory assets, current	63.9	56.6
Income Taxes Receivable, Current	13.0	8.9
Prepayments and other current assets	7.4	6.6
Property, plant & equipment	2,562.4	2,521.9
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(548.7)	(535.1)
Construction work in process	298.7	234.2
Regulatory assets, non-current	146.0	155.7
Intangible assets, net of amortization	130.9	114.0
Other Assets, Noncurrent	54.7	52.4
Liabilities, Current	489.2	350.9
Current portion of long-term debt	170.2	15.2
Accounts payable	124.3	163.9
Accrued taxes	109.9	101.6
Accrued interest	25.8	16.9
Contract with Customer, Liability, Current	20.4	12.7
Regulatory liabilities, current	16.8	18.0
Other current liabilities	21.8	22.6
Long-term debt	1,838.0	1,837.4
Deferred Tax Liabilities, Net	233.3	227.3
Taxes payable	56.0	113.4
Regulatory liabilities, non-current	180.6	182.1
Pension, retiree and other benefits	30.7	37.7
Other deferred credits	8.6	8.5
Common Stock, Value, Issued	0.0	0.0
Other paid-in capital	2,840.4	2,840.4
Accumulated other comprehensive income	(4.8)	(4.6)
Retained Earnings (Accumulated Deficit)	(2,684.3)	(2,686.9)
Liabilities	$ 2,836.4	$ 2,757.3
Subsidiaries [Member]
Common Stock, Shares Authorized	50,000,000	50,000,000
Common Stock, Shares, Outstanding	41,172,173	41,172,173
Common stock, par value (in USD per share)	$ 0.01	$ 0.01
Accounts Receivable, Allowance for Credit Loss, Current	$ 0.9	$ 0.9
Cash and cash equivalents	7.6	15.5
Accounts Receivable, after Allowance for Credit Loss, Current	100.7	93.1
Inventories	51.4	44.5
Taxes applicable to subsequent years	83.4	112.9
Regulatory assets, current	63.9	56.6
Income Taxes Receivable, Current	24.4	24.4
Prepayments and other current assets	7.5	8.1
Property, plant & equipment	3,099.7	3,063.2
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(1,107.8)	(1,098.1)
Construction work in process	295.4	230.8
Regulatory assets, non-current	146.0	155.7
Intangible assets, net of amortization	128.7	111.4
Other Assets, Noncurrent	55.0	52.9
Liabilities, Current	473.3	335.5
Current portion of long-term debt	170.2	15.2
Accounts payable	123.3	163.8
Accrued taxes	109.8	101.4
Accrued interest	13.4	4.2
Contract with Customer, Liability, Current	20.4	12.7
Regulatory liabilities, current	16.8	18.0
Other current liabilities	19.4	20.2
Long-term debt	1,012.4	1,012.3
Deferred Tax Liabilities, Net	209.0	206.1
Taxes payable	56.0	113.4
Regulatory liabilities, non-current	180.6	182.1
Pension, retiree and other benefits	30.7	37.7
Other deferred credits	4.8	4.7
Common Stock, Value, Issued	0.4	0.4
Other paid-in capital	977.5	977.4
Accumulated other comprehensive income	(27.5)	(27.9)
Retained Earnings (Accumulated Deficit)	$ 38.7	$ 29.3