Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Feb. 07, 2020
Jun. 28, 2019
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
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    
Entity Address, Address Line Three 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    
Trading Symbol MRC    
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     $ 1,423
Document Fiscal Year Focus 2019    
Amendment Flag false    
Document Fiscal Period Focus FY    
Entity Central Index Key 0001439095    
Current Fiscal Year End Date --12-31    
Common Stock [Member]      
Entity Common Stock, Shares Outstanding   81,408,974  
Restricted Stock [Member]      
Entity Common Stock, Shares Outstanding   74,055