Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 21, 2020	Jun. 30, 2019
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-13122
Entity Registrant Name	RELIANCE STEEL & ALUMINUM CO
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	95-1142616
Entity Address, Address Line One	350 South Grand Avenue, Suite 5100
Entity Address, City or Town	Los Angeles
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	90071
City Area Code	213
Local Phone Number	687-7700
Title of 12(b) Security	Common Stock, $0.001 par value
Trading Symbol	RS
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 6,260,000,000
Entity Common Stock, Shares Outstanding		66,860,589
Entity Central Index Key	0000861884
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false