Stock-Based Compensation - Narrative (Details)			12 Months Ended
Stock-Based Compensation - Narrative (Details)	May 05, 2021 shares	May 06, 2020 USD ($) shares	Dec. 31, 2025 USD ($) $ / shares shares	Dec. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2023 USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense \| $			$ 14,700,000	$ 14,000,000	$ 11,500,000
Future stock-based compensation expense \| $			$ 18,200,000
Future stock-based compensation expense, period of recognition			1 year 8 months 12 days
Time-Based Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rights description			These restricted stock units do not have any performance conditions for vesting.
Number of shares			1,501,948	1,215,132
Weighted average grant date fair value \| $ / shares			$ 14.08	$ 14.55
Performance-Based Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rights description			The overall vesting period is generally three years, with relative total shareholder return measured for the one, two and three-year periods creating effectively a “nested” 1-year, 2-year, and 3-year performance period.
Vesting period			3 years
Number of trading days to calculate average closing stock price			20 days
Number of shares			739,640	590,887
Weighted average grant date fair value \| $ / shares			$ 22.35	$ 25.93
Equity and incentive compensation 2020 plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate share limit ratio		1
Stock-based compensation, extended-term plan	May 05, 2031
Shares available for grant (in shares)		2,000,000
Shares available for grant (in shares)	2,000,000
Equity and incentive compensation 2020 plan \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized (in shares)	2,000,000	2,000,000
Share-based compensation, full value award limit	1,800,000
Equity and incentive compensation 2020 plan \| Non-employee Director \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation, maximum grant value \| $		$ 500,000
Transformation Incentive Grant Program Activity
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares			350,000
Weighted average grant date fair value \| $ / shares			$ 22.46

Metallus Inc. 10-K