Receivables From And Payables To Brokers, Dealers And Clearing Organizations (Amounts Receivable From Brokers, Dealers, And Clearing Organizations) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Due To And From Broker Dealers And Clearing Organizations [Abstract]
Deposits paid for securities borrowed	$ 350,578	$ 382,691
Receivables from clearing organizations	143,266	568,373
Securities failed to deliver	62,679	73,688
Receivables from brokers, dealers and clearing organizations, Total	$ 556,523	$ 1,024,752