| Document And Entity Information | Jul. 29, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | MRC Global inc. |
| Document, Type | 8-K |
| Document, Period End Date | Jul. 29, 2021 |
| Entity, Incorporation, State or Country Code | DE |
| Entity, File Number | 001-35479 |
| Entity, Tax Identification Number | 20-5956993 |
| Entity, Address, Address Line One | 1301 McKinney Street, 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 |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | MRC |
| Security Exchange Name | NYSE |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001439095 |
