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Investments - Carrying Value of Investments in Merchant Banking Funds (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds $ 3,675us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures $ 4,173us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
GCP I    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds 1,210us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember		 1,600us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember
GCP II    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds 887us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIiMember		 907us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIiMember
Investment in other merchant banking funds    
Investments in and Advances to Affiliates [Line Items]    
Investments in merchant banking funds $ 1,578us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_BarrowStreetCapitalIIILLCMember		 $ 1,666us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_BarrowStreetCapitalIIILLCMember