Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income attributable to Reliance	$ 80.2	$ 183.1	$ 141.9	$ 373.2
Denominator:
Weighted average shares outstanding (in shares)	63,647,000	67,045,000	64,992,000	67,086,000
Dilutive effect of stock-based awards (in shares)	935,000	932,000	923,000	865,000
Weighted average diluted shares outstanding (in shares)	64,582,000	67,977,000	65,915,000	67,951,000
Earnings per share attributable to Reliance stockholders - diluted (in dollars per share)	$ 1.24	$ 2.69	$ 2.15	$ 5.49
Earnings per share attributable to Reliance stockholders - basic (in dollars per share)	$ 1.26	$ 2.73	$ 2.18	$ 5.56
Diluted shares
Weighted average shares, respectively, primarily for RSU's and stock options, not included in the diluted calculation due to their aniti-dilutive effect			358,082	348,770