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								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues:
Revenue from contracts with customers									$ 2,678,888	$ 2,352,237
Other									48,529	19,287
Total revenue from contracts with customers									2,678,888	2,352,237
Other sources of revenue:
Interest	$ 119,876	$ 114,411	$ 128,368	$ 161,177	$ 167,087	$ 178,784	$ 187,940	$ 191,071	523,832	724,882	$ 646,449
Principal transactions									588,303	404,751	351,378
Other									26,816	33,091
Total revenues	$ 1,074,425	$ 897,122	$ 908,901	$ 937,391	$ 975,699	$ 865,716	$ 853,678	$ 819,868	3,817,839	3,514,961	3,194,957
Commissions [Member]
Revenues:
Revenue from contracts with customers									760,627	667,494	657,732
Investment Banking [Member]
Revenues:
Revenue from contracts with customers									952,308	817,421	707,670
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers									$ 917,424	$ 848,035	$ 806,175