Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 686.1	$ 690.5	$ 739.3	$ 731.6	$ 690.5	$ 731.6
Other comprehensive income (loss) before reclassifications, before income tax					1.6	(0.5)
Amounts reclassified from accumulated other comprehensive income (loss), before income tax					(2.4)	(2.3)
Amounts deferred to accumulated other comprehensive income (loss),before income tax					0.0	0.0
Tax effect	(0.2)		0.0		(0.3)	0.0
Other comprehensive income (loss), net of tax	(0.2)	(0.9)	(1.7)	(1.1)	(1.1)	(2.8)
Ending balance	690.0	686.1	736.3	739.3	690.0	736.3
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(7.7)		(6.5)	(7.7)	(6.5)
Other comprehensive income (loss) before reclassifications, before income tax					1.6	(0.5)
Amounts reclassified from accumulated other comprehensive income (loss), before income tax					0.0	0.0
Amounts deferred to accumulated other comprehensive income (loss),before income tax					0.0
Tax effect					0.0	0.0
Other comprehensive income (loss), net of tax					1.6	(0.5)
Ending balance	(6.1)		(7.0)		(6.1)	(7.0)
Pension and Postretirement Liability Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		15.4		18.9	15.4	18.9
Other comprehensive income (loss) before reclassifications, before income tax					0.0	0.0
Amounts reclassified from accumulated other comprehensive income (loss), before income tax					(2.4)	(2.3)
Amounts deferred to accumulated other comprehensive income (loss),before income tax					0.0	0.0
Tax effect					(0.3)	0.0
Other comprehensive income (loss), net of tax					(2.7)	(2.3)
Ending balance	12.7		16.6		12.7	16.6
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6.8	7.7	11.3	12.4	7.7	12.4
Other comprehensive income (loss), net of tax	(0.2)	(0.9)	(1.7)	(1.1)
Ending balance	$ 6.6	$ 6.8	$ 9.6	$ 11.3	$ 6.6	$ 9.6

Metallus Inc. 10-Q