Discontinued Operations - Income Disclosures (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Expenses:
Compensation	$ 129,631	$ 31,875	$ 236,983	$ 111,395
Payments for order flow	16,431	717	17,468	2,128
Other	11,318	5,349	30,028	19,002
Total expenses	347,965	115,990	662,285	390,317
Loss from discontinued operations, net of tax	(784)	0	(784)	0
Segment, Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues	22,114		22,114
Expenses:
Compensation	8,379		8,379
Execution and clearance fees	2,840		2,840
Payments for order flow	5,286		5,286
Other	6,873		6,873
Total expenses	23,378		23,378
Pre-tax loss from discontinued operations	(1,264)		(1,264)
Income tax benefit	480		480
Loss from discontinued operations, net of tax	$ (784)		$ (784)