Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income attributable to Reliance	$ 111.7	$ 92.2
Denominator:
Weighted average shares outstanding (in shares)	72,841,878	71,929,821
Dilutive effect of stock-based awards (in shares)	573,487	778,528
Weighted average diluted shares outstanding (in shares)	73,415,365	72,708,349
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 1.52	$ 1.27
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 1.53	$ 1.28