Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investment Banking [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	$ 381,014	$ 385,303	$ 1,300,197	$ 1,289,154
Investment Banking [Member] | Advisory Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	270,662	320,885	965,662	1,090,309
Investment Banking [Member] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	66,499	17,598	181,182	61,428
Investment Banking [Member] | Commissions and Related Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	43,853	46,820	153,353	137,417
Investment Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	14,025	12,650	39,725	37,452
Investment Management [Member] | Wealth Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	13,664	12,155	38,624	35,408
Investment Management [Member] | Institutional Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	$ 361	$ 495	$ 1,101	$ 2,044