| Document And Entity Information | Sep. 05, 2019 |
| Document And Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 5, 2019 |
| Entity Registrant Name | MRC Global inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-35479 |
| Entity Tax Identification Number | 20-5956993 |
| Entity Address, Address Line One | Fulbright Tower, 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 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common Stock, par value $0.01 |
| Trading Symbol | MRC |
| Security Exchange Name | NYSE |
| Entity Central Index Key | 0001439095 |
| Amendment Flag | false |
