General - Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
ASSETS
Non-trade receivables	$ 41.8	$ 38.1	$ 7.7
Total current assets	1,254.5	1,031.3	829.6
Property, plant and equipment - net	1,205.1	1,167.6	1,142.0
Other assets	170.4	70.4	70.2
TOTAL	2,654.9	2,269.3	2,132.8
LIABILITIES
Accrued compensation and benefits	71.8	74.4	41.6
Non-trade payables			0.0
Accrued and other current liabilities	27.0	35.6	39.2
Total current liabilities	544.7	523.6	443.6
Accrued benefits costs - less current portion	97.5	97.7	143.8
Asset retirement obligations - less current portion	70.9	75.3	85.1
Total noncurrent liabilities	825.7	815.8	851.6
SHAREHOLDERS’ EQUITY:
Noncontrolling interests	133.6	124.3	111.5
Total equity	1,284.5	929.9	837.8	$ 805.3
TOTAL	$ 2,654.9	$ 2,269.3	2,132.8
As Previously Reported
ASSETS
Non-trade receivables			0.0
Total current assets			821.9
Property, plant and equipment - net			972.2
Other assets			69.3
TOTAL			1,954.5
LIABILITIES
Accrued compensation and benefits			40.6
Non-trade payables			7.4
Accrued and other current liabilities			36.6
Total current liabilities			447.4
Accrued benefits costs - less current portion			129.3
Asset retirement obligations - less current portion			64.4
Total noncurrent liabilities			816.4
SHAREHOLDERS’ EQUITY:
Noncontrolling interests			(35.6)
Total equity			690.7	$ 662.7
TOTAL			1,954.5
Restatement Impacts
ASSETS
Non-trade receivables			7.7
Total current assets			7.7
Property, plant and equipment - net			169.8
Other assets			0.9
TOTAL			178.4
LIABILITIES
Accrued compensation and benefits			1.0
Non-trade payables			(7.4)
Accrued and other current liabilities			2.6
Total current liabilities			(3.8)
Accrued benefits costs - less current portion			14.5
Asset retirement obligations - less current portion			20.7
Total noncurrent liabilities			35.2
SHAREHOLDERS’ EQUITY:
Noncontrolling interests			147.1
Total equity			147.1
TOTAL			$ 178.4