Employee Benefits - Contributions to Reliance Sponsored Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	$ 50.0	$ 47.0	$ 45.6
Master 401(k) Plan
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	32.7	29.3	28.1
Precision Strip Retirement and Savings Plan
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	11.6	10.9	9.2
DCP
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	0.8	2.4	2.0
Other Defined Contribution Plans
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	2.2	2.0	2.0
DB Plan
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	1.3	1.2	0.3
Supplemental Executive Retirement Plans
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	$ 1.4	$ 1.2	$ 4.0