Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investment Banking [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	$ 484,120	$ 509,150	$ 919,183	$ 903,851
Investment Banking [Member] | Advisory Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	336,436	443,580	695,000	769,424
Investment Banking [Member] | Underwriting Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	93,565	16,910	114,683	43,830
Investment Banking [Member] | Commissions and Related Fees [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	54,119	48,660	109,500	90,597
Investment Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	12,953	12,419	25,700	24,802
Investment Management [Member] | Wealth Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	12,632	11,815	24,960	23,253
Investment Management [Member] | Institutional Asset Management [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer	$ 321	$ 604	$ 740	$ 1,549