Revenues from Contracts with Customers (Revenues from Contracts with Customers Disaggregated by Major Business Activity and Primary Geographic Regions) (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Commissions		$ 172,654	[1]	$ 163,920	$ 560,780	$ 484,350
Investment banking		218,134		198,790	614,637	540,247
Revenue from contracts with customers		636,899		585,587	1,875,321	1,661,442
Other		15,329		5,249	32,408	12,779
Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	137,292		93,942	359,885	247,599
Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	80,842		104,848	254,752	292,648
Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		230,782		217,628	667,496	624,066
Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Commissions		120,600	[1]	118,061	373,653	348,272
Investment banking		8,113		9,855	26,462	28,637
Revenue from contracts with customers		374,013		350,147	1,097,665	1,011,553
Other		14,535		4,615	30,104	10,619
Global Wealth Management [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	8,113		9,855	26,443	28,637
Global Wealth Management [Member] | Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]				19
Global Wealth Management [Member] | Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		230,765		217,616	667,446	624,025
Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Commissions		52,054	[1]	45,859	187,127	136,078
Investment banking		210,021		188,935	588,175	511,610
Revenue from contracts with customers		262,092		234,806	775,352	647,729
Institutional Group [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	129,179		84,087	333,442	218,962
Institutional Group [Member] | Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	80,842		104,848	254,733	292,648
Institutional Group [Member] | Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		17		12	50	41
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		794		634	2,304	2,160
Other		794		634	2,304	2,160
United States [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		583,948		546,001	1,709,297	1,558,631
United States [Member] | Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		374,013		350,147	1,097,665	1,011,553
United States [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		209,141		195,220	609,328	544,918
United States [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		794		634	2,304	2,160
United Kingdom [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		30,959		38,249	110,053	99,020
United Kingdom [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		30,959		38,249	110,053	99,020
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		21,992		1,337	55,971	3,791
Other [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		$ 21,992		$ 1,337	$ 55,971	$ 3,791

[1] Excludes revenues not derived from contracts with customers included in the Other segment.