Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 41.5	$ 25.4
Restricted cash	0.1	0.1
Accounts receivable, net	68.3	69.7
Accounts Receivable, Allowance for Credit Loss, Current	0.9	2.8
Inventories	12.4	8.8
Taxes applicable to subsequent years	19.4	78.0
Regulatory assets, current	24.5	27.5
Income Taxes Receivable, Current	18.4	17.9
Prepayments and other current assets	10.1	5.8
Total current assets	194.7	233.2
Property, plant & equipment:
Property, plant & equipment	1,952.4	1,839.3
Less: Accumulated depreciation and amortization	(455.2)	(415.7)
Property, plant and equipment, net of depreciation	1,497.2	1,423.6
Construction work in process	153.3	141.7
Total net property, plant & equipment	1,650.5	1,565.3
Other non-current assets:
Regulatory assets, non-current	191.7	193.6
Intangible assets, net of amortization	21.9	19.3
Other non-current assets	29.9	24.6
Total other non-current assets	243.5	237.5
Total assets	2,088.7	2,036.0
Current liabilities:
Current portion of long-term debt	220.2	100.2
Accounts payable	76.5	84.5
Accrued taxes	86.0	83.0
Accrued interest	18.4	16.0
Security deposits	17.8	19.4
Regulatory liabilities, current	18.9	18.0
Other current liabilities	19.8	21.0
Total current liabilities	457.6	342.1
Non-current liabilities:
Long-term debt	1,394.9	1,393.4
Deferred taxes	184.9	177.2
Taxes payable	2.4	80.4
Regulatory liabilities, non-current	214.8	218.3
Pension, retiree and other benefits	81.7	93.9
Other deferred credits	12.5	14.2
Total non-current liabilities	1,891.2	1,977.4
Commitments and contingencies
Common shareholder's equity:
Common stock	0.0	0.0
Other paid-in capital	2,468.8	2,468.8
Accumulated other comprehensive income	(11.4)	(12.3)
Retained Earnings (Accumulated Deficit)	(2,717.5)	(2,740.0)
Total common shareholder's equity	(260.1)	(283.5)
Total liabilities and shareholder's equity	$ 2,088.7	$ 2,036.0
Common Stock, Shares Authorized	1,500	1,500
Common stock, shares issued	1	1
Common Stock [Member]
Common shareholder's equity:
Total common shareholder's equity	$ 0.0	$ 0.0
Other Additional Capital [Member]
Common shareholder's equity:
Total common shareholder's equity	2,468.8	2,468.8
Subsidiaries [Member]
Current assets:
Cash and cash equivalents	29.8	11.7
Restricted cash	0.1	0.1
Accounts receivable, net	68.5	70.2
Accounts Receivable, Allowance for Credit Loss, Current	0.9	2.8
Inventories	12.4	8.8
Taxes applicable to subsequent years	19.4	77.6
Regulatory assets, current	24.5	27.5
Income Taxes Receivable, Current	30.3	32.5
Prepayments and other current assets	11.4	6.4
Total current assets	196.4	234.8
Property, plant & equipment:
Property, plant & equipment	2,535.8	2,437.3
Less: Accumulated depreciation and amortization	(1,056.4)	(1,032.1)
Property, plant and equipment, net of depreciation	1,479.4	1,405.2
Construction work in process	149.6	138.8
Total net property, plant & equipment	1,629.0	1,544.0
Other non-current assets:
Regulatory assets, non-current	191.7	193.6
Intangible assets, net of amortization	20.8	18.3
Other non-current assets	29.7	24.0
Total other non-current assets	242.2	235.9
Total assets	2,067.6	2,014.7
Current liabilities:
Current portion of long-term debt	140.2	20.2
Accounts payable	76.3	85.5
Accrued taxes	87.3	82.7
Accrued interest	6.0	2.6
Security deposits	17.6	19.1
Regulatory liabilities, current	18.9	18.0
Other current liabilities	14.9	25.0
Total current liabilities	361.2	253.1
Non-current liabilities:
Long-term debt	574.0	573.9
Deferred taxes	184.5	172.1
Taxes payable	2.4	80.3
Regulatory liabilities, non-current	214.8	218.3
Pension, retiree and other benefits	81.7	93.9
Other deferred credits	6.0	6.4
Total non-current liabilities	1,063.4	1,144.9
Commitments and contingencies
Par value common shares (in USD per share)	$ 0.01	$ 0.01
Common shareholder's equity:
Common stock	$ 0.4	$ 0.4
Other paid-in capital	697.5	714.4
Accumulated other comprehensive income	(39.2)	(42.1)
Retained Earnings (Accumulated Deficit)	(15.7)	(56.0)
Total common shareholder's equity	643.0	616.7
Total liabilities and shareholder's equity	$ 2,067.6	$ 2,014.7
Common Stock, Shares, Outstanding	41,172,173	41,172,173
Common Stock, Shares Authorized	50,000,000	50,000,000
Subsidiaries [Member] | Common Stock [Member]
Common shareholder's equity:
Total common shareholder's equity	$ 0.4
Subsidiaries [Member] | Other Additional Capital [Member]
Common shareholder's equity:
Total common shareholder's equity	$ 697.5