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			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Revenues:
Revenue from contracts with customers	[1]	$ 532,144			$ 1,605,149
Other	[1]	4,394			11,908
Total revenue from contracts with customers		532,165			1,605,150
Other sources of revenue:
Interest		169,760		$ 117,862	461,915		$ 327,766
Principal transactions		83,152		87,010	269,918		299,570
Other		1,733			6,649
Total revenues		786,810		738,794	2,343,632		2,172,512
Commissions [Member]
Revenues:
Revenue from contracts with customers		158,016		162,612	490,693		510,150
Investment Banking [Member]
Revenues:
Revenue from contracts with customers		169,012		181,904	506,437		494,017
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers		$ 200,743	[1]	$ 179,848	$ 596,112	[1]	$ 515,501

[1] Excludes revenue included in the Other segment.