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Investments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
GCP I    
Fair Value, Measurement Inputs, Disclosure [Line Items]    
Investment related to noncontrolling interests $ 0.1us-gaap_PartnersCapitalAttributableToNoncontrollingInterest
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember
$ 0.1us-gaap_PartnersCapitalAttributableToNoncontrollingInterest
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIMember
GCP II    
Fair Value, Measurement Inputs, Disclosure [Line Items]    
Investment related to noncontrolling interests 0.6us-gaap_PartnersCapitalAttributableToNoncontrollingInterest
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIiMember
0.6us-gaap_PartnersCapitalAttributableToNoncontrollingInterest
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_GcpIiMember
Barrow Street Capital III    
Fair Value, Measurement Inputs, Disclosure [Line Items]    
Other commitment $ 0.3us-gaap_OtherCommitment
/ us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis
= ghl_BarrowStreetCapitalIIILLCMember