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, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Receivable/Payable:
Clearing organizations and other	$ 1,095,025	$ 750,440
Assets segregated or held in separate accounts under federal or other regulations (1)	0	304,294
Securities failed to deliver	93,808	202,517
Total Receivable	1,188,833	1,257,251
Clearing organizations and other	350,627	425,196
Securities failed to receive	325,462	48,912
Total Payable	$ 676,089	$ 474,108