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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Revenue from contracts with customers	$ 585,587	$ 532,144	$ 1,661,442	$ 1,605,149
Other	5,249	4,394	12,779	11,908
Total revenue from contracts with customers	585,587	532,165	1,661,442	1,605,150
Other sources of revenue:
Interest	178,784	169,760	557,795	461,915
Principal transactions	97,847	83,152	298,343	269,918
Other	3,498	1,733	21,682	6,649
Total revenues	865,716	786,810	2,539,262	2,343,632
Commissions [Member]
Revenues:
Revenue from contracts with customers	163,920	158,016	484,350	490,693
Investment Banking [Member]
Revenues:
Revenue from contracts with customers	198,790	169,012	540,247	506,437
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers	$ 217,628	$ 200,743	$ 624,066	$ 596,112