Revenues from Contracts with Customers (Schedule of Total Revenues Separated between Revenues from Contracts with Customers and Other Sources of Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues:
Revenue from contracts with customers	$ 847,931	$ 633,108
Other	16,882	4,767
Total revenue from contracts with customers	847,931	633,108
Other sources of revenue:
Interest	127,540	161,177
Principal transactions	165,006	138,666
Other	8,752	4,440
Total revenues	1,149,229	937,391
Commissions [Member]
Revenues:
Revenue from contracts with customers	213,614	211,098
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	339,288	179,468
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 278,147	$ 237,775