Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income attributable to Reliance	$ 61.7	$ 49.5	$ 100.9	$ 92.2	$ 68.6	$ 51.4	$ 90.2	$ 101.3	$ 304.3	$ 311.5	$ 371.5
Denominator for basic earnings per share:
Weighted average shares outstanding (in shares)									72,362,513	74,096,349	77,682,943
Effect of dilutive securities:
Dilutive effect of stock-based awards (in shares)									758,405	805,715	932,996
Denominator:
Weighted average diluted shares outstanding (in shares)									73,120,918	74,902,064	78,615,939
Net income per share attributable to Reliance shareholders - diluted (in dollars per share)	$ 0.84	$ 0.68	$ 1.38	$ 1.27	$ 0.94	$ 0.69	$ 1.20	$ 1.30	$ 4.16	$ 4.16	$ 4.73
Net income per share attributable to Reliance shareholders - basic (in dollars per share)	$ 0.85	$ 0.68	$ 1.39	$ 1.28	$ 0.96	$ 0.70	$ 1.21	$ 1.31	$ 4.21	$ 4.20	$ 4.78