Quarterly Financial Information (Unaudited) (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
Summary of the quarterly results of operations
Net sales	$ 2,061.6	$ 2,185.2	$ 2,203.9	$ 2,162.7	$ 2,026.2	$ 2,286.2	$ 2,423.7	$ 2,614.4	$ 8,613.4	$ 9,350.5	$ 10,451.6
Cost of sales	1,447.7	1,530.6	1,518.8	1,526.0	1,444.2	1,647.9	1,767.8	1,943.7	6,023.1	6,803.6	7,830.6
Gross profit	613.9	654.6	685.1	636.7	582.0	638.3	655.9	670.7
Net income	62.9	50.6	102.1	93.5	68.9	52.8	91.6	102.9	309.1	316.2	376.3
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
Basic earnings per common share (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
Diluted earnings per common share (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