Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income attributable to Reliance (in dollars)	$ 92.3	$ 95.5	$ 96.5	$ 87.2	$ 61.8	$ 95.1	$ 81.0	$ 83.7	$ 371.5	$ 321.6	$ 403.5
Denominator for basic earnings per share:
Weighted average shares									77,682,943	76,844,912	75,216,955
Effect of dilutive securities:
Stock options, restricted stock, and RSUs (in shares)									932,996	801,280	477,257
Denominator for diluted earnings per share:
Adjusted weighted average shares and assumed conversions									78,615,939	77,646,192	75,694,212
Net income per share attributable to Reliance shareholders - diluted (in dollars per share)	$ 1.18	$ 1.21	$ 1.22	$ 1.11	$ 0.79	$ 1.22	$ 1.05	$ 1.09	$ 4.73	$ 4.14	$ 5.33
Net income per share attributable to Reliance shareholders - basic (in dollars per share)	$ 1.19	$ 1.23	$ 1.24	$ 1.13	$ 0.80	$ 1.23	$ 1.06	$ 1.10	$ 4.78	$ 4.19	$ 5.36
Diluted shares
Shares reserved for issuance upon exercise of outstanding stock options or vesting of restricted shares respectively, not included in the diluted calculation due to their anti-dilutive effect									24,823	192,293	2,234,568