Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of the quarterly results of operations
Net sales	$ 2,447.8	$ 2,685.9	$ 2,883.5	$ 2,956.6	$ 2,814.0	$ 2,974.5	$ 2,988.9	$ 2,757.1	$ 10,973.8	$ 11,534.5	$ 9,721.0
Cost of sales	1,653.6	1,871.2	2,029.9	2,089.7	2,104.2	2,140.2	2,071.4	1,937.2	7,644.4	8,253.0	6,933.2
Gross profit	794.2	814.7	853.6	866.9	709.8	834.3	917.5	819.9
Net income	166.3	163.9	184.3	191.6	87.3	150.3	233.1	171.1	706.1	641.8	621.0
Net income attributable to Reliance	$ 165.6	$ 162.7	$ 183.1	$ 190.1	$ 85.6	$ 148.3	$ 230.8	$ 169.0	$ 701.5	$ 633.7	$ 613.4
Diluted earnings per common share (in dollars per share)	$ 2.44	$ 2.40	$ 2.69	$ 2.80	$ 1.22	$ 2.03	$ 3.16	$ 2.30	$ 10.34	$ 8.75	$ 8.34
Basic earnings per common share (in dollars per share)	$ 2.48	$ 2.44	$ 2.73	$ 2.83	$ 1.23	$ 2.06	$ 3.19	$ 2.32	$ 10.49	$ 8.85	$ 8.42