Revenues from Contracts with Customers (Schedule of Total Revenues Broken Out by Revenues from Contracts with Customers and Other Sources of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenues:
Revenue from contracts with customers	$ 876,248	$ 605,314	$ 1,724,179	$ 1,238,422
Other	8,357	12,312	25,239	17,079
Total revenue from contracts with customers	876,248	605,314	1,724,179	1,238,422
Other sources of revenue:
Interest	133,591	128,368	261,131	289,545
Principal transactions	152,597	166,017	317,603	304,683
Other	4,878	9,202	13,630	13,642
Total revenues	1,167,314	908,901	2,316,543	1,846,292
Commissions [Member]
Revenues:
Revenue from contracts with customers	195,579	177,028	409,193	388,126
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	376,443	217,035	715,731	396,503
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 295,869	$ 198,939	$ 574,016	$ 436,714