| Cover | Oct. 06, 2020 |
| Schedule of Capitalization, Equity [Line Items] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 6, 2020 |
| Entity File Number | 001-38615 |
| Entity Registrant Name | FORUM MERGER II CORPORATION |
| Entity Central Index Key | 0001741231 |
| Entity Tax Identification Number | 82-5457906 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1615 South Congress Avenue |
| Entity Address, Address Line Two | Suite 103 |
| Entity Address, City or Town | Delray Beach |
| Entity Address, State or Province | FL |
| Entity Address, Postal Zip Code | 33445 |
| City Area Code | (212) |
| Local Phone Number | 739-7860 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | false |
| Entity Information, Former Legal or Registered Name | Not Applicable |
| Units, each consisting of one share of Class A common stock and one redeemable warrant | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | Units, each consisting of one share of Class A common stock and one redeemable warrant |
| Trading Symbol | FMCIU |
| Security Exchange Name | NASDAQ |
| Class A common stock, par value $0.0001 per share | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | Class A common stock, par value $0.0001 per share |
| Trading Symbol | FMCI |
| Security Exchange Name | NASDAQ |
| Warrants, each exercisable for one share of Class A common stock | |
| Schedule of Capitalization, Equity [Line Items] | |
| Title of 12(b) Security | Warrants, each exercisable for one share of Class A common stock |
| Trading Symbol | FMCIW |
| Security Exchange Name | NASDAQ |
