Fair Value of Financial Assets and Liabilities - Quantitative Information about Level 3 Fair Value Measurements on Recurring Basis (Detail) $ in Thousands	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Fair Value by Fair Value Hierarchy Level [Extensible List]	us-gaap:FairValueInputsLevel3Member	us-gaap:FairValueInputsLevel3Member
Liabilities, fair value	$ 36,515	$ 42,159
Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Valuation Technique [Extensible List]	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember
Minimum [Member] | Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	0.068	0.092
Minimum [Member] | Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	0.39	0.70
Maximum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Valuation Technique [Extensible List]	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember
Maximum [Member] | Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	0.103	0.103
Maximum [Member] | Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	1	1
Weighted Average [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Valuation Technique [Extensible List]	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember	bgcp:PresentValueOfExpectedPaymentsApproachValuationTechniqueMember
Weighted Average [Member] | Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	0.095	0.098
Weighted Average [Member] | Probability of Meeting Earnout and Contingencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability, Measurement Input	0.789	0.902