Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator:
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
Denominator:
Weighted average shares outstanding (in shares)									71,621	72,851	72,363
Dilutive effect of stock-based awards (in shares)									820	688	758
Weighted average diluted shares outstanding (in shares)									72,441	73,539	73,121
Earnings per share attributable to Reliance stockholders - diluted (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
Earnings per share attributable to Reliance stockholders - basic (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