Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 22, 2020
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	001-35479
Entity Registrant Name	MRC GLOBAL INC.
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	Fulbright Tower
Entity Address, Address Line Two	1301 McKinney Street, Suite 2300
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Tax Identification Number	20-5956993
Entity Address, Postal Zip Code	77010
City Area Code	877
Local Phone Number	294-7574
Title of 12(b) Security	Common Stock, par value $0.01
Trading Symbol	MRC
Security Exchange Name	NYSE
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
Document Fiscal Year Focus	2020
Amendment Flag	false
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001439095
Current Fiscal Year End Date	--12-31
Common Stock [Member]
Entity Common Stock, Shares Outstanding		81,995
Restricted Stock [Member]
Entity Common Stock, Shares Outstanding		74,055