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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues:
Revenue from contracts with customers	$ 636,899	$ 585,587	$ 1,875,321	$ 1,661,442
Other	15,329	5,249	32,408	12,779
Total revenue from contracts with customers	636,899	585,587	1,875,321	1,661,442
Other sources of revenue:
Interest	114,411	178,784	403,956	557,795
Principal transactions	140,883	97,847	445,566	298,343
Other	4,929	3,498	18,571	21,682
Total revenues	897,122	865,716	2,743,414	2,539,262
Commissions [Member]
Revenues:
Revenue from contracts with customers	172,654	163,920	560,780	484,350
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	218,134	198,790	614,637	540,247
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 230,782	$ 217,628	$ 667,496	$ 624,066