v3.26.1
Average Annual Total Returns
 12 Months Ended 60 Months Ended 120 Months Ended
May 01, 2026
Dec. 31, 2025
Dec. 31, 2025
Dec. 31, 2025
S&P 500® Index (total return) (reflects no deduction of fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] S&P 500® Index(reflects no deduction of fees, expenses or taxes)      
Average Annual Return, Percent   17.88% 14.42% 14.82%
Russell 3000® Index (reflects no deduction of fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [1] Russell 3000® Index      
Average Annual Return, Percent [1]   17.15% 13.15% 14.29%
NASDAQ Bank Index (reflects no deduction of fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] NASDAQ Bank Index2      (reflects no deduction of fees, expenses or taxes)      
Average Annual Return, Percent   0.35% 6.93% 6.97%
MSCI EAFE Index (reflects no deduction of fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] MSCI EAFE Index      (reflects no deduction of fees, expenses or taxes)      
Average Annual Return, Percent   31.22% 8.92% 7.22% [2]
Russell 2500TM Index (reflects no deduction of fees, expenses or taxes)        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [3] Russell 2500TM Index2     (reflects no deduction of fees, expenses or taxes)      
Average Annual Return, Percent [3]   11.91% 7.26% 10.40%
RMB Fund, Class A        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   6.86% 8.23% 11.03%
Performance Inception Date Feb. 01, 2017      
RMB Fund, Class A | After Taxes on Distributions        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions      
Average Annual Return, Percent   4.60% 6.65% 8.41%
RMB Fund, Class A | After Taxes on Distributions and Sales        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [4] Total Return After Taxes on Distributions and Sale of Fund Shares1      
Average Annual Return, Percent [4]   5.64% 6.30% 8.15%
RMB Fund, Class C        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   10.70% 8.53% 10.77%
Performance Inception Date Feb. 01, 2017      
RMB Fund, Class I        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [5] Total Return Before Taxes      
Average Annual Return, Percent [5]   12.80% 9.62% 11.85%
Performance Inception Date Feb. 01, 2017      
RMB Mendon Financial Services Fund, Class A        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   7.20% 10.70% 8.78%
Performance Inception Date Feb. 01, 2017      
RMB Mendon Financial Services Fund, Class A | After Taxes on Distributions        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions      
Average Annual Return, Percent   5.65% 9.31% 7.72%
RMB Mendon Financial Services Fund, Class A | After Taxes on Distributions and Sales        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions and Sale of Fund Shares      
Average Annual Return, Percent   5.27% 8.24% 6.91%
RMB Mendon Financial Services Fund, Class C        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   11.00% 11.00% 8.52%
Performance Inception Date Feb. 01, 2017      
RMB Mendon Financial Services Fund, Class I        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [6] Total Return Before Taxes      
Average Annual Return, Percent [6]   13.12% 12.11% 9.66%
Performance Inception Date Feb. 01, 2017      
RMB International Fund, Class I        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   32.74% 6.41% 3.93% [2]
Performance Inception Date Dec. 27, 2017      
RMB International Fund, Class I | After Taxes on Distributions        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions      
Average Annual Return, Percent   32.44% 6.14% 3.75% [2]
RMB International Fund, Class I | After Taxes on Distributions and Sales        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions and Sale of Fund Shares      
Average Annual Return, Percent   19.96% 5.13% 3.19% [2]
RMB SMID Cap Fund Class I        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return Before Taxes      
Average Annual Return, Percent   2.55% 6.55% 10.56%
RMB SMID Cap Fund Class I | After Taxes on Distributions        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] Total Return After Taxes on Distributions      
Average Annual Return, Percent   (1.44%) 4.16% 7.71%
RMB SMID Cap Fund Class I | After Taxes on Distributions and Sales        
Prospectus [Line Items]        
Average Annual Return, Label [Optional Text] [7] Total Return After Taxes on Distributions and Sale of Fund Shares1      
Average Annual Return, Percent [7]   4.41% 5.01% 7.98%
[1] The returns of this additional index provide an additional comparison for the Fund’s performance against that of a broad measure of the U.S. equity market.
[2] Class I shares commenced investment operations on December 27, 2017.
[3] 2    The returns of this additional index provide an additional comparison for the Fund's performance against that of an index that more closely reflects the market segment in which the Fund invests.
[4]
1The “Total Return After Taxes on Distributions and Sale of Fund Shares” can be higher than other return figures when a capital loss occurs upon the redemption of Fund Shares. If realized losses occur upon the sale of Fund shares, the capital loss is recorded as a tax benefit, which increases the return.
[5] Class I shares commenced investment operations on February 1, 2017. Performance shown prior to February 1, 2017 for the Class I shares reflects the performance of Class A shares, excluding the front-end sales charge that is applicable to Class A shares but not applicable to Class I shares. Class I shares are also not subject to the distribution and shareholder service (12b-1) fees applicable to Class A shares, which reduce the performance shown for the Class I shares prior to February 1, 2017.
[6] Class I shares commenced investment operations on February 1, 2017. Performance shown prior to February 1, 2017 for the Class I shares reflects the performance of Class A shares, excluding the front-end sales charge that is applicable to Class A shares but not applicable to Class I shares. Class I shares are also not subject to the distribution and shareholder service (12b-1) fees applicable to Class A shares, which reduce the performance shown for the Class I shares prior to February 1, 2017
[7] The “Total Return After Taxes on Distributions and Sale of Fund Shares” can be higher than other return figures when a capital loss occurs upon the redemption of Fund shares. If realized losses occur upon the sale of Fund shares, the capital loss is recorded as a tax benefit, which increases the return.