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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Significant Accounting Policies [Line Items]      
Client reimbursements $ 2.0us-gaap_ReimbursementRevenue $ 0.7us-gaap_ReimbursementRevenue  
Bad debt expense 0.2us-gaap_ProvisionForDoubtfulAccounts 0us-gaap_ProvisionForDoubtfulAccounts  
Long term receivables related to private equity and real estate capital advisory engagements 35.2us-gaap_AccountsReceivableNetNoncurrent   37.3us-gaap_AccountsReceivableNetNoncurrent
Installments period 3 years    
Decrease to goodwill from translation adjustments 9.3us-gaap_GoodwillTranslationAdjustments    
Depreciation and amortization of property and equipment Depreciation is computed using the straight-line method over the life of the assets. Amortization of leasehold improvements is computed using the straight-line method over the lesser of the life of the asset or the remaining term of the lease.    
Aircraft      
Significant Accounting Policies [Line Items]      
Estimated useful lives of fixed assets (in years) 7 years    
Equipment      
Significant Accounting Policies [Line Items]      
Estimated useful lives of fixed assets (in years) 5 years    
Furniture and Fixtures      
Significant Accounting Policies [Line Items]      
Estimated useful lives of fixed assets (in years) 7 years    
Leasehold Improvements      
Significant Accounting Policies [Line Items]      
Estimated useful lives of fixed assets (in years) 10 years    
Description of estimated useful lives Lesser of 10 years or the remaining lease term    
Capital advisory engagements      
Significant Accounting Policies [Line Items]      
Interest income on long term receivables $ 0.2ghl_InterestIncomeOnLongTermReceivables
/ us-gaap_StatementBusinessSegmentsAxis
= ghl_AdvisoryServicesMember
$ 0.2ghl_InterestIncomeOnLongTermReceivables
/ us-gaap_StatementBusinessSegmentsAxis
= ghl_AdvisoryServicesMember
 
Restricted Stock      
Significant Accounting Policies [Line Items]      
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period 5 years