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				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Commissions		$ 195,579	[1]	$ 177,028	[1]	$ 409,193	[1]	$ 388,126
Investment banking		376,443		217,035		715,731		396,503
Revenue from contracts with customers		876,248		605,314		1,724,179		1,238,422
Other		8,357		12,312		25,239		17,079
Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	169,778		119,197		378,584		222,593
Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	206,665		97,838		337,147		173,910
Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		295,869		198,939		574,016		436,714
Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Commissions		139,653	[1]	116,156	[1]	287,158	[1]	253,053
Investment banking		11,898		8,016		25,447		18,349
Revenue from contracts with customers		455,820		334,661		909,907		723,652
Other		8,422		11,568		23,346		15,569
Global Wealth Management [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	11,898		8,016		25,447		18,330
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		295,847		198,921		573,956		436,681
Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Commissions		55,926	[1]	60,875	[1]	122,035	[1]	135,073
Investment banking		364,545		209,019		690,284		378,154
Revenue from contracts with customers		420,493		269,912		812,379		513,260
Institutional Group [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	157,880		111,181		353,137		204,263
Institutional Group [Member] | Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	206,665		97,838		337,147		173,891
Institutional Group [Member] | Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		22		18		60		33
Other [Member]
Disaggregation Of Revenue [Line Items]
Commissions	[1]			(3)
Revenue from contracts with customers		(65)		741		1,893		1,510
Other		(65)		744		1,893		1,510
United States [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		800,249		537,546		1,560,142		1,125,349
United States [Member] | Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		455,820		334,661		909,907		723,652
United States [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		344,494		202,144		648,342		400,187
United States [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		(65)		741		1,893		1,510
United Kingdom [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		45,824		48,331		91,215		79,094
United Kingdom [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		45,824		48,331		91,215		79,094
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		8,864		8,553		21,765		17,872
Other [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		8,864		8,553		21,765		17,872
Canada [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		21,311		10,884		51,057		16,107
Canada [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		$ 21,311		$ 10,884		$ 51,057		$ 16,107

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