Condensed Consolidated Balance Sheets - USD ($)	Jun. 30, 2025	Dec. 31, 2024
CURRENT ASSETS:
Cash and cash equivalents	$ 70,100,000	$ 54,000,000.0
Accounts Receivable, after Allowance for Credit Loss, Current	137,100,000	106,800,000
Inventories	73,800,000	70,500,000
Taxes applicable to subsequent years	67,800,000	137,100,000
Regulatory assets, current	97,200,000	82,000,000.0
Income Taxes Receivable, Current	21,700,000	27,100,000
Prepayments and other current assets	10,200,000	7,500,000
Total current assets	477,900,000	485,000,000.0
Property, Plant and Equipment, Net	2,702,800,000	2,571,400,000
Assets, Total	3,521,000,000	3,417,400,000
Current liabilities:
Current portion of long-term debt	310,200,000	704,400,000
Accounts payable	112,400,000	132,100,000
Accrued taxes	126,700,000	115,400,000
Accrued interest	14,000,000.0	22,800,000
Contract with Customer, Liability, Current	10,800,000	14,200,000
Regulatory liabilities, current	6,800,000	10,300,000
Other current liabilities	40,000,000.0	35,000,000.0
Total current liabilities	620,900,000	1,034,200,000
Non-current liabilities:
Long-term debt	1,426,100,000	1,425,600,000
Deferred Tax Liabilities, Net	222,800,000	249,600,000
Taxes payable	68,300,000	137,500,000
Regulatory liabilities, non-current	173,900,000	169,400,000
Pension, retiree and other benefits	23,500,000	30,700,000
Other deferred credits	7,800,000	8,300,000
Total non-current liabilities	1,922,400,000	2,021,100,000
Commitments and contingencies
Redeemable Noncontrolling Interest, Equity, Carrying Amount	584,300,000	0
Common shareholder's equity:
Other paid-in capital	3,048,400,000	3,040,500,000
Accumulated other comprehensive income	14,700,000	(5,300,000)
Retained Earnings (Accumulated Deficit)	(2,669,700,000)	(2,673,100,000)
Total common shareholder's equity	393,400,000	362,100,000
Total liabilities and shareholder's equity	3,521,000,000	3,417,400,000
Liabilities	2,543,300,000	3,055,300,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(607,600,000)	(571,700,000)
Regulatory assets, non-current	131,400,000	140,000,000.0
Intangible assets, net of amortization	172,700,000	175,100,000
Accounts Receivable, Allowance for Credit Loss, Current	10,100,000	6,100,000
Assets, Noncurrent	3,043,100,000	2,932,400,000
Other Assets, Noncurrent	36,200,000	45,900,000
Subsidiaries [Member]
CURRENT ASSETS:
Assets, Total	$ 3,462,600,000	$ 3,355,200,000