Business Held for Sale and Discontinued Operations - Balance Sheet Disclosures (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets within discontinued operations:
Receivable from brokers, dealers and clearing organizations	$ 1,366,974	$ 868,805
Total assets	108,804	0
Liabilities
Financial instruments sold, not yet purchased, at fair value	1,512,983	1,445,437
Financial instruments sold under agreements to repurchase	545,000	355,000
Payable to brokers, dealers and clearing organizations	635,914	378,724
Total liabilities	84,535	0
Segment, Discontinued Operations [Member]
Assets within discontinued operations:
Financial instruments owned, at fair value, including securities pledged	0
Receivable from brokers, dealers and clearing organizations	93,443
Other assets	15,361
Total assets	108,804
Liabilities
Financial instruments sold, not yet purchased, at fair value	0
Financial instruments sold under agreements to repurchase	16,251
Payable to brokers, dealers and clearing organizations	34,866
Accrued compensation expense	16,611
Accrued expenses and other liabilities	16,807
Total liabilities	84,535
Assets Held-for-sale [Member]
Assets within discontinued operations:
Financial instruments owned, at fair value, including securities pledged		138,689
Receivable from brokers, dealers and clearing organizations		281,178
Other assets		29,642
Total assets		449,509
Liabilities
Financial instruments sold, not yet purchased, at fair value		199,167
Financial instruments sold under agreements to repurchase		111,487
Payable to brokers, dealers and clearing organizations		23,852
Accrued compensation expense		12,140
Accrued expenses and other liabilities		11,015
Total liabilities		$ 357,661