Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income attributable to Reliance	$ 97.3	$ 49.5	$ 312.0	$ 242.6
Denominator:
Weighted average shares outstanding (in shares)	72,908,979	72,542,873	72,881,000	72,282,537
Dilutive effect of stock-based awards (in shares)	708,500	737,924	630,427	752,401
Weighted average diluted shares outstanding (in shares)	73,617,479	73,280,797	73,511,427	73,034,938
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 1.32	$ 0.68	$ 4.24	$ 3.32
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 1.33	$ 0.68	$ 4.28	$ 3.36