Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income attributable to Reliance	$ 148.3	$ 97.3	$ 548.1	$ 312.0
Denominator:
Weighted average shares outstanding (in shares)	71,940,253	72,908,979	72,364,121	72,881,000
Dilutive effect of stock-based awards (in shares)	1,040,977	708,500	772,461	630,427
Weighted average diluted shares outstanding (in shares)	72,981,230	73,617,479	73,136,582	73,511,427
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 2.03	$ 1.32	$ 7.49	$ 4.24
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 2.06	$ 1.33	$ 7.57	$ 4.28