Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income attributable to Reliance	$ 61.7	$ 190.1
Denominator:
Weighted average shares outstanding (in shares)	66,337,000	67,127,000
Dilutive effect of stock-based awards (in shares)	911,000	799,000
Weighted average diluted shares outstanding (in shares)	67,248,000	67,926,000
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 0.92	$ 2.80
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 0.93	$ 2.83
Diluted shares
Weighted average shares, respectively, primarily for RSU's and stock options, not included in the diluted calculation due to their aniti-dilutive effect	701,053	682,660