| Document and Entity Information | Aug. 03, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001333274 |
| Document Type | 8-K |
| Document Period End Date | Aug. 3, 2021 |
| Entity Registrant Name | MERCER INTERNATIONAL INC. |
| Entity Incorporation State Country Code | WA |
| Entity File Number | 000-51826 |
| Entity Tax Identification Number | 47-0956945 |
| Entity Address, Address Line One | Suite 1120 |
| Entity Address, Address Line Two | 700 West Pender Street |
| Entity Address, City or Town | Vancouver |
| Entity Address, State or Province | BC |
| Entity Address, Country | CA |
| Entity Address, Postal Zip Code | V6C 1G8 |
| City Area Code | (604) |
| Local Phone Number | 684-1099 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $1.00 per share |
| Trading Symbol | MERC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
