| Shareholder Report | 6 Months Ended |
| Sep. 30, 2024 USD ($) Holding |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | FORUM FUNDS |
| Entity Central Index Key | 0000315774 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Sep. 30, 2024 |
| C000081163 | |
| Shareholder Report [Line Items] | |
| Fund Name | Beck Mack + Oliver Partners Fund |
| Trading Symbol | BMPEX |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains important information about the Beck Mack + Oliver Partners Fund for the period of April 1, 2024, to September 30, 2024. |
| Shareholder Report Annual or Semi-Annual | Semi-Annual Shareholder Report |
| Additional Information [Text Block] | This semi-annual shareholder report contains important information about the Beck Mack + Oliver Partners Fund for the period of April 1, 2024, to September 30, 2024. You can find additional information about the Fund at https://www.beckmack.com/products. You can also request this information by contacting us at (800) 943-6786. |
| Additional Information Phone Number | (800) 943-6786 |
| Additional Information Website | https://www.beckmack.com/products |
| Expenses [Text Block] | What were the Fund's costs for the last six months? (based on a hypothetical $10,000 investment) Fund Name Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment Beck Mack + Oliver Partners Fund $51 1.00% |
| Expenses Paid, Amount | $ 51 |
| Expense Ratio, Percent | 1% |
| Factors Affecting Performance [Text Block] | How did the Fund perform in the last six months? During the six-month semi-annual period ended September 30, 2024 (the “Semi-Annual Period”), the Beck Mack + Oliver Partners Fund (the “Partners Fund”) returned +4.27% net of fees and expenses, resulting in a net asset value of $25.18. By comparison, the S&P 500 Index (the “S&P 500”), which is the Partners Fund’s principal benchmark, returned +10.42%. We believe that one factor influencing the relative performance of the Partners Fund during the Semi-Annual Period was the salience of market cap with respect to total returns. Among the constituent stocks of the S&P 500, those with the 25 highest total returns during the Semi-Annual Period had an average market cap of $227 billion, while those with the 25 lowest total returns during the Semi-Annual Period had an average market cap of $28 billion. Within the Partners Fund, stocks that had a positive contribution to investment performance during the Semi-Annual Period had an average market cap of $400 billion, while those that had a negative contribution to investment performance during the Semi-Annual Period had an average market cap of $32 billion. The S&P 500 is a market cap-weighted index, such that stocks with larger market caps have higher weightings in the index. The S&P 500 Equal Weight Index, which assigns an equal weight to each S&P 500 constituent, returned +6.71% during the Semi-Annual Period. Radnet, which operates radiology centers, returned +42.60% and contributed +2.48% to investment performance during the Semi-Annual Period. Fortrea, which as a contract research organization runs outsourced clinical trials for pharmaceutical and biotechnology customers, returned -50.17% and contributed -1.74% to investment performance during the Semi-Annual Period. |
| Performance Past Does Not Indicate Future [Text] | The Fund’s past performance is not a good predictor of how the Fund will perform in the future. |
| Line Graph [Table Text Block] | Date Beck Mack + Oliver Partners Fund S&P 500 ® 09/30/14 $10,000 $10,000 12/31/14 $9,606 $10,493 03/31/15 $9,178 $10,593 06/30/15 $9,444 $10,622 09/30/15 $7,929 $9,939 12/31/15 $8,143 $10,638 03/31/16 $8,072 $10,782 06/30/16 $8,188 $11,047 09/30/16 $8,656 $11,472 12/31/16 $9,000 $11,911 03/31/17 $9,318 $12,633 06/30/17 $9,509 $13,024 09/30/17 $10,090 $13,607 12/31/17 $10,608 $14,511 03/31/18 $10,508 $14,401 06/30/18 $10,854 $14,896 09/30/18 $11,727 $16,044 12/31/18 $9,009 $13,875 03/31/19 $10,218 $15,769 06/30/19 $10,690 $16,447 09/30/19 $10,681 $16,727 12/31/19 $11,923 $18,244 03/31/20 $8,463 $14,668 06/30/20 $10,581 $17,682 09/30/20 $10,535 $19,260 12/31/20 $12,526 $21,600 03/31/21 $15,401 $22,934 06/30/21 $17,485 $24,895 09/30/21 $17,871 $25,040 12/31/21 $19,248 $27,801 03/31/22 $18,073 $26,522 06/30/22 $14,905 $22,252 09/30/22 $14,005 $21,165 12/31/22 $15,186 $22,766 03/31/23 $15,912 $24,473 06/30/23 $17,767 $26,612 09/30/23 $17,574 $25,741 12/31/23 $20,088 $28,750 03/31/24 $22,192 $31,785 06/30/24 $21,586 $33,147 09/30/24 $23,139 $35,098 |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on Fund distributions or redemption of Fund shares. |
| Performance Table Market Index Changed [Text Block] | |
| Updated Performance Information Location [Text Block] | |
| AssetsNet | $ 72,348,024 |
| Holdings Count | Holding | 25 |
| Advisory Fees Paid, Amount | $ 155,875 |
| InvestmentCompanyPortfolioTurnover | 2% |
| Additional Fund Statistics [Text Block] | Fund Statistics Total Net Assets $72,348,024 # of Portfolio Holdings 25 Portfolio Turnover Rate 2% Investment Advisory Fees (Net of fees waived)
$155,875 |
| Holdings [Text Block] | Sector Weightings (% total investments) Value Value Money Market Fund 1.4% Materials 2.4% Energy 2.8% Consumer, Cyclical 3.3% Consumer Discretionary 3.8% Communication Services 6.0% Information Technology 8.2% Industrials 12.1% Health Care 17.7% Financials 42.3% |
| Largest Holdings [Text Block] | Top Ten Holdings (% total net assets) Apollo Global Management, Inc. 8.80% Blackstone, Inc., Class A 7.83% RadNet, Inc. 7.67% Microsoft Corp. 6.25% Alphabet, Inc., Class C 6.01% Fiserv, Inc. 5.71% Ashtead Group PLC 5.24% Arthur J Gallagher & Co. 5.06% Enstar Group, Ltd. 4.00% Zurn Elkay Water Solutions Corp. 3.98% |
