v3.25.2
Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended 139 Months Ended 147 Months Ended 151 Months Ended 171 Months Ended 254 Months Ended 291 Months Ended
Sep. 26, 2025
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
Dec. 31, 2024
 [9]
Dec. 31, 2024
 [1]
Dec. 31, 2024
 [5]
Dec. 31, 2024
 [1]
Dec. 31, 2024
 [1]
ARBITRAGE FUND | Class R                    
Prospectus [Line Items]                    
Average Annual Return, Percent   2.62% 2.73% 2.59%           3.70%
Performance Inception Date Sep. 18, 2000                  
ARBITRAGE FUND | Class R | After Taxes on Distributions                    
Prospectus [Line Items]                    
Average Annual Return, Percent   2.35% 1.75% 1.71%           2.78%
ARBITRAGE FUND | Class R | After Taxes on Distributions and Sales                    
Prospectus [Line Items]                    
Average Annual Return, Percent   1.68% 1.75% 1.68%           2.61%
ARBITRAGE FUND | Class I                    
Prospectus [Line Items]                    
Average Annual Return, Percent   2.95% 2.99% 2.84%         3.03%  
Performance Inception Date Oct. 17, 2003                  
ARBITRAGE FUND | Class C                    
Prospectus [Line Items]                    
Average Annual Return, Percent   0.92% 1.97% 1.82%     1.54%      
Performance Inception Date Jun. 01, 2012                  
ARBITRAGE FUND | Class A                    
Prospectus [Line Items]                    
Average Annual Return, Percent   (0.19%) 2.22% 2.32% 2.24% [1]          
Performance Inception Date Jun. 01, 2013                  
ARBITRAGE FUND | Standard & Poor's 500® Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [2]   25.02% 14.53% 13.10%           7.91%
ARBITRAGE FUND | ICE BofA U.S. 3-Month Treasury Bill Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [3]   5.25% 2.46% 1.77%           1.80%
ARBITRAGE FUND | Bloomberg U.S. Aggregate Bond Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [4]   1.25% (0.33%) 1.35%           3.79%
WATER ISLAND EVENT-DRIVEN FUND | Class R                    
Prospectus [Line Items]                    
Average Annual Return, Percent   1.91% 4.02% 2.39%       2.40%    
Performance Inception Date Oct. 01, 2010                  
WATER ISLAND EVENT-DRIVEN FUND | Class R | After Taxes on Distributions                    
Prospectus [Line Items]                    
Average Annual Return, Percent   1.91% 3.98% 2.20%       1.93%    
WATER ISLAND EVENT-DRIVEN FUND | Class R | After Taxes on Distributions and Sales                    
Prospectus [Line Items]                    
Average Annual Return, Percent   1.13% 3.10% 1.77%       1.67%    
WATER ISLAND EVENT-DRIVEN FUND | Class I                    
Prospectus [Line Items]                    
Average Annual Return, Percent   2.27% 4.31% 2.66%       2.66%    
Performance Inception Date Oct. 01, 2010                  
WATER ISLAND EVENT-DRIVEN FUND | Class A                    
Prospectus [Line Items]                    
Average Annual Return, Percent   (1.35%) 3.37% 2.07% 1.98% [5]          
Performance Inception Date Jun. 01, 2013                  
WATER ISLAND EVENT-DRIVEN FUND | Standard & Poor's 500® Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [6]   25.02% 14.53% 13.10%       14.35%    
WATER ISLAND EVENT-DRIVEN FUND | ICE BofA U.S. 3-Month Treasury Bill Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [7]   5.25% 2.46% 1.77%       1.26%    
WATER ISLAND EVENT-DRIVEN FUND | Bloomberg U.S. Aggregate Bond Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [8]   1.25% (0.33%) 1.35%       1.95%    
WATER ISLAND CREDIT OPPORTUNITIES FUND | Class R                    
Prospectus [Line Items]                    
Average Annual Return, Percent   5.23% 3.69% 2.99%   2.96%        
Performance Inception Date Oct. 01, 2012                  
WATER ISLAND CREDIT OPPORTUNITIES FUND | Class R | After Taxes on Distributions                    
Prospectus [Line Items]                    
Average Annual Return, Percent   3.32% 2.23% 1.70%   1.65%        
WATER ISLAND CREDIT OPPORTUNITIES FUND | Class R | After Taxes on Distributions and Sales                    
Prospectus [Line Items]                    
Average Annual Return, Percent   3.07% 2.20% 1.72%   1.68%        
WATER ISLAND CREDIT OPPORTUNITIES FUND | Class I                    
Prospectus [Line Items]                    
Average Annual Return, Percent   5.50% 3.96% 3.26%   3.21%        
Performance Inception Date Oct. 01, 2012                  
WATER ISLAND CREDIT OPPORTUNITIES FUND | Class A                    
Prospectus [Line Items]                    
Average Annual Return, Percent   1.77% 3.03% 2.65% 2.63% [9]          
Performance Inception Date Jun. 01, 2013                  
WATER ISLAND CREDIT OPPORTUNITIES FUND | ICE BofA U.S. 3-Month Treasury Bill Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [10]   5.25% 2.46% 1.77%   1.45%        
WATER ISLAND CREDIT OPPORTUNITIES FUND | Bloomberg U.S. Aggregate Bond Index                    
Prospectus [Line Items]                    
Average Annual Return, Percent [11]   1.25% (0.33%) 1.35%   1.43%        
[1] The inception date for Class R shares is September 18, 2000, the inception date for Class I shares is October 17, 2003, the inception date for Class C shares is June 1, 2012, and the inception date for the Class A shares is June 1, 2013. The "Since Inception" returns reflected for the Standard & Poor's 500®​ Index, the ICE BofA U.S. 3-Month Treasury Bill Index and the Bloomberg U.S. Aggregate Bond Index are based on the inception date for Class R shares.
[2] Due to new regulatory requirements, effective May 31, 2024, the Standard & Poor's 500®​ Index became the Fund's broad-based securities market index. The Standard & Poor's 500®​ Index serves as the Fund's regulatory index and provides a broad measure of market performance. The Standard and Poor's 500®​ Index, or simply the S&P 500, is a stock market index tracking the performance of 500 large companies listed on stock exchanges in the U.S.
[3] The ICE BofA U.S. 3-Month Treasury Bill Index tracks the performance of the U.S. Treasury Bills publicly issued in the U.S. domestic market with a remaining term to final maturity of less than 3 months.
[4] The Bloomberg U.S. Aggregate Bond Index is a market value-weighted index of investment grade fixed-rated debt issues, including government, corporate, asset-backed and mortgage-backed securities with a maturity of one year or more.
[5] The inception date for Class R shares and Class I shares is October 1, 2010 and the inception date for the Class A shares is June 1, 2013. The "Since Inception" returns reflected for the Standard & Poor's 500®​ Index, the ICE BofA U.S. 3-Month Treasury Bill Index and the Bloomberg U.S. Aggregate Bond Index are based on the inception date for Class R shares.
[6] Due to new regulatory requirements, effective May 31, 2024, the Standard & Poor's 500®​ Index became the Fund's broad-based securities market index. The Standard & Poor's 500®​ Index serves as the Fund's regulatory index and provides a broad measure of market performance. The Standard and Poor's 500®​ Index, or simply the S&P 500, is a stock market index tracking the performance of 500 large companies listed on stock exchanges in the U.S.
[7] The ICE BofA U.S. 3-Month Treasury Bill Index tracks the performance of the U.S. Treasury Bills publicly issued in the U.S. domestic market with a remaining term to final maturity of less than 3 months.
[8] The Bloomberg U.S. Aggregate Bond Index is a market value-weighted index of investment grade fixed-rated debt issues, including government, corporate, asset-backed and mortgage-backed securities with a maturity of one year or more.
[9] The inception date for Class R shares and Class I shares is October 1, 2012. The inception date for the Class A shares is June 1, 2013. The "Since Inception" returns reflected for the Bloomberg U.S. Aggregate Bond Index and the ICE BofA U.S. 3-Month Treasury Bill Index are based on the inception date for Class R and Class I shares.
[10] The ICE BofA U.S. 3-Month Treasury Bill Index is the Fund's additional index. The ICE BofA U.S. 3-Month Treasury Bill Index tracks the performance of the U.S. Treasury Bills publicly issued in the U.S. domestic market with a remaining term to final maturity of less than 3 months.
[11] Due to new regulatory requirements, effective May 31, 2024, the Bloomberg U.S. Aggregate Bond Index became the Fund's broad-based securities market index. The Bloomberg U.S. Aggregate Bond Index serves as the Fund's regulatory index and provides a broad measure of market performance. The Bloomberg U.S. Aggregate Bond Index is a market value-weighted index of investment grade fixed-rated debt issues, including government, corporate, asset-backed and mortgage-backed securities with a maturity of one year or more.