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Investments - Carrying Value of Investments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds $ 4,173us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures $ 11,745us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
GCP I    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds 1,600us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember		 2,257us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember
GCP II    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds 907us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GreenhillCapitalPartnersIIMember		 7,690us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GreenhillCapitalPartnersIIMember
Barrow Street III    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds $ 1,666us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_BarrowStreetCapitalIIILLCMember		 $ 1,798us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_BarrowStreetCapitalIIILLCMember