| Document and Entity Information | Apr. 30, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001439095 |
| Document Type | 8-K |
| Document Period End Date | Apr. 30, 2021 |
| Entity Registrant Name | MRC GLOBAL INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35479 |
| Entity Tax Identification Number | 20-5956993 |
| 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 Address, Postal Zip Code | 77010 |
| City Area Code | (877) |
| Local Phone Number | 294-7574 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.01 |
| Trading Symbol | MRC |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
