Related Party Transactions (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Fees receivable		$ 138,234		$ 138,234		$ 126,358
Interest Income, Related Party			$ 100	100	$ 100
Due from related parties		23,480		23,480		26,749
Redeemable non-controlling interests, Related Party		0		0		391,275
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds		0	(6,848)	(32,924)	(4,187)
Investment banking		194,341	77,292	503,351	272,103
Adjustments | Management Reclassifications
Related Party Transaction [Line Items]
Investment banking	[1],[2]	(9,190)	(7,859)	(28,573)	(17,729)
Employees
Related Party Transaction [Line Items]
Redeemable non-controlling interests, Related Party		192,000		192,000		36,000
Net income (loss) attributable to non-controlling interests in consolidated subsidiaries and investment funds		6,700	3,600	10,700	4,700
Employee Loans
Related Party Transaction [Line Items]
Due from employees		11,300		11,300		14,900
Forgivable Loan Balances		4,900		4,900		7,100
Amortization on Forgivable Loans		800	1,000	$ 2,800	$ 2,800
Employee Loans | Minimum
Related Party Transaction [Line Items]
Forgivable Loans, Vesting Period				1 year
Employee Loans | Maximum
Related Party Transaction [Line Items]
Forgivable Loans, Vesting Period				3 years
Affiliated Entity
Related Party Transaction [Line Items]
Fees receivable		22,600		$ 22,600		20,500
Investor
Related Party Transaction [Line Items]
Due to Affiliate		200		200		300
Other Funds
Related Party Transaction [Line Items]
Due from related parties		7,400		7,400		5,300
Starboard Value LP
Related Party Transaction [Line Items]
Effective interest rate					5.00%
Interest Income, Related Party		100	$ 100	200	$ 300
Due from related parties		$ 4,800		$ 4,800		$ 6,500
Finance Period				5 years

[1] Economic Income (Loss) presents expenses reimbursed from clients and affiliates within their respective expense category but is included as a part of revenues under US GAAP.

[2] Economic Income (Loss) presents underwriting expenses net of investment banking revenues.