Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2020
Significant Accounting Policies [Line Items]
Contract with customer, liability		$ 4,800		$ 4,800		$ 3,900
Revenues		56,045	$ 87,036	170,930	$ 194,315
Accounts receivable, credit loss expense (reversal)		0	0	(100)	$ 400
Retained earnings		33,195		$ 33,195		69,093
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.
Operating lease right-of-use asset		78,688		$ 78,688		28,346
Operating lease obligations		84,213		$ 84,213		$ 30,750
Accounting Standards Update [Extensible List]				us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member
Revision of Prior Period, Accounting Standards Update, Adjustment
Significant Accounting Policies [Line Items]
Operating lease right-of-use asset	$ 38,100
Operating lease obligations	38,100
Deferred rent	3,200
Increase (decrease) in operating liabilities	$ 3,200
Minimum
Significant Accounting Policies [Line Items]
Amortization period of deferred cash retention awards				3 years
Maximum
Significant Accounting Policies [Line Items]
Amortization period of deferred cash retention awards				5 years
Equipment
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets				5 years
Furniture and Fixtures
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets				7 years
Leasehold Improvements
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets				10 years
Restricted Stock | Minimum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period				3 years
Restricted Stock | Maximum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period				5 years
Capital advisory engagements
Significant Accounting Policies [Line Items]
Contract with customer, liability, revenue recognized				$ 2,200	$ 4,400
Cumulative effect of the change in accounting principle related to credit losses
Significant Accounting Policies [Line Items]
Retained earnings							$ (100)
Primary Capital Advisory Engagements
Significant Accounting Policies [Line Items]
Long term receivables related to private equity and real estate capital advisory engagements		5,200		$ 5,200		$ 10,500
Installments period				3 years
Reimbursement Revenue
Significant Accounting Policies [Line Items]
Revenues		$ 500	$ 1,500	$ 2,400	$ 4,000