Quarterly Financial Information (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of the quarterly results of operations
Net sales	$ 2,576.7	$ 2,705.1	$ 2,616.8	$ 2,553.0	$ 2,306.7	$ 2,443.5	$ 2,448.3	$ 2,025.3	$ 10,451.6	$ 9,223.8	$ 8,442.3
Cost of sales	1,954.4	2,026.9	1,943.5	1,905.8	1,702.5	1,800.5	1,826.7	1,496.5	7,830.6	6,826.2	6,235.4
Gross profit	622.3	678.2	673.3	647.2	604.2	643.0	621.6	528.8
Net income	93.7	96.9	97.8	87.9	62.3	96.0	81.9	84.5	376.3	324.7	408.3
Net income attributable to Reliance	$ 92.3	$ 95.5	$ 96.5	$ 87.2	$ 61.8	$ 95.1	$ 81.0	$ 83.7	$ 371.5	$ 321.6	$ 403.5
Diluted earnings per common share attributable to Reliance shareholders (in dollars per share)	$ 1.18	$ 1.21	$ 1.22	$ 1.11	$ 0.79	$ 1.22	$ 1.05	$ 1.09	$ 4.73	$ 4.14	$ 5.33
Basic earnings per common share attributable to Reliance shareholders (in dollars per share)	$ 1.19	$ 1.23	$ 1.24	$ 1.13	$ 0.80	$ 1.23	$ 1.06	$ 1.10	$ 4.78	$ 4.19	$ 5.36