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Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
Document Type dei_DocumentType 497
Document Period End Date dei_DocumentPeriodEndDate Dec. 31, 2019
Entity Registrant Name dei_EntityRegistrantName RMB INVESTORS TRUST
Entity Central Index Key dei_EntityCentralIndexKey 0000030126
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate May 07, 2020
Document Effective Date dei_DocumentEffectiveDate May 01, 2020
Prospectus Date rr_ProspectusDate May 01, 2020
Entity Inv Company Type dei_EntityInvCompanyType N-1A
RMB Mendon Financial Long/Short Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return [Heading] rr_RiskReturnHeading RMB Mendon Financial Long/Short Fund
Supplement to Prospectus [Text Block] rr_SupplementToProspectusTextBlock

RMB FUNDS

RMB Mendon Financial Long/Short Fund
(the “Fund”)

Supplement dated May 1, 2020
to the Fund’s Summary Prospectus and Prospectus dated May 1, 2020

IMPORTANT NOTICE REGARDING INVESTMENT POLICIES
The “Principal Investment Strategies” section of the Fund’s Summary Prospectus, the “Principal Investment Strategies” section within the Fund’s Summary section of the Prospectus, and the “Additional Information About the Funds’ Investments” section of the Fund’s Prospectus is supplemented to include the following:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
As a result of recent market disruptions arising from the rapid and escalating spread of COVID-19, the Fund has adopted a temporary defensive position as provided in the Prospectus. Accordingly, the Fund is expected to maintain a high level of cash and other temporary defensive instruments in its portfolio during this period of market disruption. The Fund will not be achieving its investment objective to the extent it takes such a defensive position.
Supplement Closing [Text Block] ck0000030126_SupplementClosingTextBlock
Please retain this supplement with your Summary Prospectus and Prospectus for future reference.
RMB Mendon Financial Long/Short Fund | Class A  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol RMBFX
RMB Mendon Financial Long/Short Fund | Class C  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol RMBCX
RMB Mendon Financial Long/Short Fund | Class I  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol RMBIX