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, 2020		Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Commissions		$ 177,028	[1]	$ 164,981	$ 388,126	$ 320,430
Investment banking		217,035		179,617	396,503	341,457
Revenue from contracts with customers		605,314		559,517	1,238,422	1,075,855
Other		12,312		3,748	17,079	7,530
Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	119,197		96,712	222,593	153,657
Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	97,838		82,905	173,910	187,800
Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		198,939		211,171	436,714	406,438
Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Commissions		116,156	[1]	120,284	253,053	230,211
Investment banking		8,016		10,559	18,349	18,782
Revenue from contracts with customers		334,661		345,548	723,652	661,406
Other		11,568		3,549	15,569	6,004
Global Wealth Management [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	8,016		10,559	18,330	18,782
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		198,921		211,156	436,681	406,409
Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Commissions		60,875	[1]	44,697	135,073	90,219
Investment banking		209,019		169,058	378,154	322,675
Revenue from contracts with customers		269,912		213,770	513,260	412,923
Institutional Group [Member] | Capital Raising [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	111,181		86,153	204,263	134,875
Institutional Group [Member] | Advisory Fees [Member]
Disaggregation Of Revenue [Line Items]
Investment banking	[1]	97,838		82,905	173,891	187,800
Institutional Group [Member] | Asset Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		18		15	33	29
Other [Member]
Disaggregation Of Revenue [Line Items]
Commissions	[1]	(3)
Revenue from contracts with customers		741		199	1,510	1,526
Other		744		199	1,510	1,526
United States [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		537,546		518,705	1,125,349	1,012,630
United States [Member] | Global Wealth Management [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		334,661		345,548	723,652	661,406
United States [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		202,144		172,958	400,187	349,698
United States [Member] | Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		741		199	1,510	1,526
United Kingdom [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		48,331		39,668	79,094	60,771
United Kingdom [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		48,331		39,668	79,094	60,771
Other [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		19,437		1,144	33,979	2,454
Other [Member] | Institutional Group [Member]
Disaggregation Of Revenue [Line Items]
Revenue from contracts with customers		$ 19,437		$ 1,144	$ 33,979	$ 2,454

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