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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Revenue from contracts with customers	$ 605,314	$ 559,517	$ 1,238,422	$ 1,075,855
Other	12,312	3,748	17,079	7,530
Total revenue from contracts with customers	605,314	559,517	1,238,422	1,075,855
Other sources of revenue:
Interest	128,368	187,940	289,545	379,011
Principal transactions	166,017	96,464	304,683	200,496
Other	9,202	9,757	13,642	18,184
Total revenues	908,901	853,678	1,846,292	1,673,546
Commissions [Member]
Revenues:
Revenue from contracts with customers	177,028	164,981	388,126	320,430
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	217,035	179,617	396,503	341,457
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 198,939	$ 211,171	$ 436,714	$ 406,438