| Shareholder Report | 12 Months Ended |
| Dec. 31, 2024 USD ($) Holding |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | FORUM FUNDS |
| Entity Central Index Key | 0000315774 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Dec. 31, 2024 |
| C000012470 | |
| Shareholder Report [Line Items] | |
| Fund Name | Polaris Global Value Fund |
| Trading Symbol | PGVFX |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains important information about the Polaris Global Value Fund for the period of January 1, 2024, to December 31, 2024. |
| Shareholder Report Annual or Semi-Annual | Annual Shareholder Report |
| Additional Information [Text Block] | This annual shareholder report contains important information about the Polaris Global Value Fund for the period of January 1, 2024, to December 31, 2024. You can find additional information about the Fund at https://polarisfunds.com/resources/. You can also request this information by contacting us at (888) 263-5594. |
| Additional Information Phone Number | (888) 263-5594 |
| Additional Information Website | https://polarisfunds.com/resources/ |
| Expenses [Text Block] | What were the Fund's costs for the last year? (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 Polaris Global Value Fund $102 0.99% |
| Expenses Paid, Amount | $ 102 |
| Expense Ratio, Percent | 0.99% |
| Factors Affecting Performance [Text Block] | How did the Fund perform in the last year? While the fourth quarter faltered slightly, annual results for the MSCI World Index landed in resoundingly positive territory, up 18.67% for the year. A more comprehensive breakout of Index results showed that growth and U.S. stocks prevailed while small-cap, value and non-U.S. companies lagged. This proved out among the large-cap technology and consumer stocks such as Nvidia, Amazon.com and Apple, which drove Index results. And the Index skewed heavily toward U.S. stocks (at nearly 74%), while the more diversified Polaris Global Value Fund held only 42% of U.S. stocks. That diversity extended throughout the portfolio, with more small/mid-cap names and markedly more value plays than the Index, resulting in underperformance for the year, with the Polaris Global Value Fund returning 5.34%. At the sector level, the Fund had double-digit gains from financials, consumer staples and utilities sectors, partially offset by negative results in two sectors: information technology and consumer discretionary. From a country perspective, the U.S. contributed most to the Fund's gains, but a significant underweight limited impact. Holdings in Germany, Ireland, Singapore and a handful of out-of-benchmark countries posted double-digit gains. Detractors were notable in countries suffering geopolitical turmoil, including France, South Korea and Canada. Top Contributors Top Detractors Allison Transmission Holdings, Inc. Williams Cos., Inc. United Therapeutics Corp. SLM Corp. JPMorgan Chase & Co. Teleperformance SE CVS Health Corp. Magna International, Inc. OpenText Corp. |
| 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 Polaris Global Value Fund MSCI World Index 12/31/14 $10,000 $10,000 03/31/15 $10,517 $10,231 06/30/15 $10,607 $10,263 09/30/15 $9,673 $9,396 12/31/15 $10,155 $9,913 03/31/16 $10,227 $9,878 06/30/16 $9,891 $9,978 09/30/16 $10,798 $10,463 12/31/16 $11,340 $10,657 03/31/17 $11,880 $11,337 06/30/17 $12,497 $11,793 09/30/17 $12,964 $12,364 12/31/17 $13,676 $13,045 03/31/18 $13,336 $12,878 06/30/18 $13,513 $13,101 09/30/18 $13,745 $13,753 12/31/18 $11,945 $11,908 03/31/19 $13,185 $13,394 06/30/19 $13,604 $13,930 09/30/19 $13,445 $14,005 12/31/19 $14,668 $15,203 03/31/20 $10,012 $12,003 06/30/20 $11,880 $14,326 09/30/20 $12,398 $15,462 12/31/20 $15,644 $17,621 03/31/21 $17,347 $18,488 06/30/21 $17,815 $19,920 09/30/21 $17,627 $19,919 12/31/21 $18,052 $21,465 03/31/22 $17,218 $20,359 06/30/22 $14,991 $17,064 09/30/22 $13,603 $16,008 12/31/22 $15,883 $17,571 03/31/23 $16,329 $18,930 06/30/23 $16,607 $20,222 09/30/23 $16,237 $19,522 12/31/23 $18,229 $21,751 03/31/24 $19,307 $23,683 06/30/24 $19,119 $24,306 09/30/24 $20,397 $25,853 12/31/24 $19,202 $25,812 |
| Average Annual Return [Table Text Block] | One Year Five Year Ten Year Polaris Global Value Fund 5.34% 5.53% 6.74% MSCI World Index 18.67% 11.17% 9.95% |
| 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 | $ 408,925,326 |
| Holdings Count | Holding | 97 |
| Advisory Fees Paid, Amount | $ 3,359,527 |
| InvestmentCompanyPortfolioTurnover | 17% |
| Additional Fund Statistics [Text Block] | Fund Statistics Total Net Assets $408,925,326 # of Portfolio Holdings 97 Portfolio Turnover Rate 17% Investment Advisory Fees (Net of fees waived)
$3,359,527 |
| Holdings [Text Block] | Country Weightings (% total investments)* Value Value United States 42.1% Japan 8.4% France 7.1% South Korea 6.1% Germany 5.9% Canada 5.4% United Kingdom 4.6% Switzerland 3.7% Norway 3.5% Ireland 3.3% Sweden 2.1% Netherlands 1.4% Puerto Rico 1.4% Italy 1.3% Singapore 1.3% Taiwan 0.9% China 0.6% Chile 0.5% Colombia 0.4% Russia 0.0% * excluding cash equivalents |
| Largest Holdings [Text Block] | Top Ten Holdings (% total investments)* Allison Transmission Holdings, Inc. 2.14% Smurfit WestRock PLC 1.68% United Therapeutics Corp. 1.59% Capital One Financial Corp. 1.59% Cullen/Frost Bankers, Inc. 1.57% JPMorgan Chase & Co. 1.57% Northern Trust Corp. 1.56% SLM Corp. 1.56% Methanex Corp. 1.55% The Carlyle Group, Inc. 1.51% * excluding cash equivalents |
