Receivable from and Payable to Brokers, Dealers and Clearing Organizations - Schedule of Amounts Receivable from and Payable to Brokers, Dealers and Clearing Organizations (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Receivable/Payable:
Clearing organizations and other	$ 476,488	$ 1,073,480
Financial instruments purchased under agreement to resell	21,845	21,545
Securities failed to deliver	191,958	93,808
Total Receivable	690,291	1,188,833
Clearing organizations and other	461,000	350,627
Securities failed to receive	68,748	325,462
Total Payable	$ 529,748	$ 676,089