Employee Benefits - Supplemental Bonus Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	$ 37.3	$ 34.1	$ 33.8
Master Plan
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	21.4	21.5	18.4
Other Defined Contribution Plans
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	7.9	7.0	7.8
Employee Stock Ownership Plan
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	1.5	1.8	1.4
Deferred Compensation Plan
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	0.6	0.6	0.6
Supplemental Executive Retirement Plans
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	3.5	2.6	3.2
Defined Benefit Plans
Contributions to Company Sponsored Retirement Plans
Company's expense (credit)for Reliance-sponsored retirement plans	$ 2.4	$ 0.6	$ 2.4