Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income attributable to Reliance	$ 169.0	$ 111.7
Denominator:
Weighted average shares outstanding (in shares)	72,818,336	72,841,878
Dilutive effect of stock-based awards (in shares)	632,029	573,487
Weighted average diluted shares outstanding (in shares)	73,450,365	73,415,365
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 2.30	$ 1.52
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 2.32	$ 1.53