Employee Benefits - Supplemental Bonus Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	$ 42.1	$ 45.0	$ 45.9
Master Plan
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	22.7	28.1	25.0
Other defined contribution plans
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	9.6	9.4	9.0
ESOP
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans		1.9	1.8
Reliance Deferred Compensation Plan
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	5.0	0.9	0.9
SERPs
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	3.3	2.9	6.5
Defined Benefit Plans
Contributions to Company Sponsored Retirement Plans
Company's expense for Reliance-sponsored retirement plans	$ 1.5	$ 1.8	$ 2.7