Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Contract with customer, liability	$ 5,300,000		$ 5,300,000				$ 5,600,000
Advisory revenues	86,550,000	$ 86,495,000	192,705,000	$ 261,315,000
Provision for doubtful accounts	0	0	$ 400,000	300,000
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	28,127,000		$ 28,127,000				0
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
Revenue | Maximum
Significant Accounting Policies [Line Items]
Percent of revenues			1.00%
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
Restricted Stock | Minimum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period (years)			3 years
Restricted Stock | Maximum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period (years)			5 years
Capital advisory engagements
Significant Accounting Policies [Line Items]
Contract with customer, liability, revenue recognized			$ 4,400,000	9,000,000
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Operating lease right-of-use asset						$ 38,100,000
Deferred rent						$ (3,200,000)
Primary Capital Advisory Engagements
Significant Accounting Policies [Line Items]
Long term receivables related to private equity and real estate capital advisory engagements	14,000,000		$ 14,000,000				20,000,000
Installments period (in years)			3 years
Reimbursement Revenue
Significant Accounting Policies [Line Items]
Advisory revenues	$ 1,500,000	$ 2,100,000	$ 4,000,000	$ 5,400,000
Retained earnings
Significant Accounting Policies [Line Items]
Cumulative effect of the change in accounting principle related to revenue recognition					$ 0		$ 0	$ 0		$ (7,645,000)
Retained earnings | Accounting Standards Update 2014-09
Significant Accounting Policies [Line Items]
Cumulative effect of the change in accounting principle related to revenue recognition									$ (7,600,000)