Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of the quarterly results of operations
Net sales	$ 2,814.0	$ 2,974.5	$ 2,988.9	$ 2,757.1	$ 2,376.4	$ 2,450.1	$ 2,475.2	$ 2,419.3	$ 11,534.5	$ 9,721.0	$ 8,613.4
Cost of sales	2,104.2	2,140.2	2,071.4	1,937.2	1,697.8	1,764.6	1,773.1	1,697.7	8,253.0	6,933.2	6,023.1
Gross profit	709.8	834.3	917.5	819.9	678.6	685.5	702.1	721.6
Net income	87.3	150.3	233.1	171.1	303.8	99.0	104.8	113.4	641.8	621.0	309.1
Net income attributable to Reliance	$ 85.6	$ 148.3	$ 230.8	$ 169.0	$ 301.4	$ 97.3	$ 103.0	$ 111.7	$ 633.7	$ 613.4	$ 304.3
Diluted earnings per common share (in dollars per share)	$ 1.22	$ 2.03	$ 3.16	$ 2.30	$ 4.09	$ 1.32	$ 1.40	$ 1.52	$ 8.75	$ 8.34	$ 4.16
Basic earnings per common share (in dollars per share)	$ 1.23	$ 2.06	$ 3.19	$ 2.32	$ 4.14	$ 1.33	$ 1.41	$ 1.53	$ 8.85	$ 8.42	$ 4.21