Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income attributable to Reliance	$ 103.0	$ 100.9	$ 214.7	$ 193.1
Denominator:
Weighted average shares outstanding (in shares)	72,891,406	72,372,056	72,866,779	72,150,938
Dilutive effect of stock-based awards (in shares)	609,295	740,752	591,391	759,640
Weighted average diluted shares outstanding (in shares)	73,500,701	73,112,808	73,458,170	72,910,578
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 1.40	$ 1.38	$ 2.92	$ 2.65
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 1.41	$ 1.39	$ 2.95	$ 2.68