Business Segments - Income (Loss) from Continuing Operations Before Income Taxes ("Pre-Tax Earnings") and Total Assets by Segment (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 467,819,000	$ 130,602,000	$ 698,123,000	$ 425,241,000
Pre-tax earnings	119,854,000	14,612,000	35,838,000	34,924,000
Total assets	7,194,995,000	1,773,305,000	7,194,995,000	1,773,305,000
Discontinued operations assets		6,100,000,000		6,100,000,000
Market Making [Member]
Segment Reporting Information [Line Items]
Revenues	240,110,000	107,672,000	455,678,000	383,203,000
Pre-tax earnings	47,853,000	8,214,000	55,608,000	37,976,000
Total assets	3,882,763,000	1,507,881,000	3,882,763,000	1,507,881,000
Global Execution Services [Member]
Segment Reporting Information [Line Items]
Revenues	91,366,000	9,258,000	113,701,000	27,248,000
Pre-tax earnings	(16,354,000)	(1,120,000)	(21,248,000)	(3,839,000)
Total assets	1,426,824,000	25,934,000	1,426,824,000	25,934,000
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	136,343,000	13,672,000	128,744,000	14,790,000
Pre-tax earnings	88,355,000	7,518,000	1,478,000	787,000
Total assets	$ 1,885,408,000	$ 239,490,000	$ 1,885,408,000	$ 239,490,000