Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income attributable to Reliance	$ 49.5	$ 51.4	$ 242.6	$ 242.9
Denominator for basic earnings per share:
Weighted average shares outstanding (in shares)	72,542,873	73,316,356	72,282,537	74,903,472
Effect of dilutive securities:
Dilutive effect of stock-based awards (in shares)	737,924	819,837	752,401	770,124
Denominator:
Weighted average diluted shares outstanding (in shares)	73,280,797	74,136,193	73,034,938	75,673,596
Net income per share attributable to Reliance shareholders - diluted (in dollars per share)	$ 0.68	$ 0.69	$ 3.32	$ 3.21
Net income per share attributable to Reliance shareholders - basic (in dollars per share)	$ 0.68	$ 0.70	$ 3.36	$ 3.24