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			6 Months Ended
		Jun. 30, 2018		Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Revenues:
Revenue from contracts with customers	[1]	$ 531,349			$ 1,073,005
Other	[1]	3,796			7,514
Total revenue from contracts with customers		531,329			1,072,985
Other sources of revenue:
Interest		154,421		$ 108,951	292,155		$ 209,904
Principal transactions		88,984		95,703	186,766		212,560
Other		5,277			4,916
Total revenues		780,011		742,291	1,556,822		1,433,718
Commissions [Member]
Revenues:
Revenue from contracts with customers		166,902		172,264	332,677		347,538
Investment Banking [Member]
Revenues:
Revenue from contracts with customers		161,063		185,261	337,425		312,113
Asset Management and Service Fees [Member]
Revenues:
Revenue from contracts with customers		$ 199,568	[1]	$ 172,914	$ 395,369	[1]	$ 335,653

[1] Excludes revenue included in the Other segment.