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, 2018	Mar. 31, 2017
Revenues from contracts with customers:
Commissions	$ 165,775	$ 175,274
Investment banking	176,362	126,852
Asset management and service fees	195,801	162,739
Other	3,718
Total revenue from contracts with customers	541,656
Other sources of revenue:
Interest	137,734	100,953
Principal transactions	97,782	116,857
Other	(361)
Total revenues	$ 776,811	$ 691,427