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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues:
Revenue from contracts with customers	$ 880,472	$ 636,899	$ 2,604,651	$ 1,875,321
Other	5,092	15,329	30,331	32,408
Total revenue from contracts with customers	880,472	636,899	2,604,651	1,875,321
Other sources of revenue:
Interest	141,844	114,411	402,975	403,956
Principal transactions	118,977	140,883	436,580	445,566
Other	13,668	4,929	27,298	18,571
Total revenues	1,154,961	897,122	3,471,504	2,743,414
Commissions [Member]
Revenues:
Revenue from contracts with customers	189,239	172,654	598,432	560,780
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	372,279	218,134	1,088,010	614,637
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 313,862	$ 230,782	$ 887,878	$ 667,496