CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 798,712,000	$ 427,631,000
Cash and cash equivalents segregated under federal and other regulations	216,442,000	0
Financial instruments owned, at fair value, including securities pledged to counterparties that had the right to deliver or repledge of $961,479 at September 30, 2013 and $52,261 at December 31, 2012:
Equities	2,156,466,000	378,933,000
Listed options	288,227,000	92,305,000
Debt securities	79,284,000	183,637,000
Total financial instruments owned, at fair value	2,523,977,000	654,875,000
Collateralized agreements:
Securities borrowed	1,370,921,000	52,261,000
Receivable from brokers, dealers and clearing organizations	1,330,113,000	142,969,000
Fixed assets and leasehold improvements, less accumulated depreciation and amortization	161,865,000	83,341,000
Investments	125,889,000	248,438,000
Goodwill	18,398,000	4,645,000
Intangible assets, less accumulated amortization	192,045,000	46,123,000
Deferred tax asset	169,619,000	4,180,000
Assets within discontinued operations	6,098,299,000	0
Other assets	287,015,000	23,073,000
Total assets	13,293,295,000	1,687,536,000
Financial instruments sold, not yet purchased, at fair value:
Equities	1,848,728,000	423,740,000
Listed options	229,722,000	69,757,000
Debt securities	79,057,000	19,056,000
Other financial instruments	5,431,000	0
Total financial instruments sold, not yet purchased, at fair value	2,162,938,000	512,553,000
Collateralized financings:
Securities loaned	543,451,000	0
Financial instruments sold under agreements to repurchase	595,029,000	0
Total collateralized financings	1,138,480,000	0
Payable to brokers, dealers and clearing organizations	666,178,000	24,185,000
Payable to customers	486,136,000	0
Accrued compensation expense	130,158,000	27,728,000
Accrued expenses and other liabilities	220,648,000	118,068,000
Capital lease obligations	12,453,000	24,191,000
Liabilities within discontinued operations	6,006,024,000	0
Short-term debt	235,000,000	0
Long-term debt	722,259,000	15,000,000
Total liabilities	11,780,274,000	721,725,000
Redeemable preferred member's equity	0	311,139,000
Members' equity	0	654,672,000
Equity
Additional paid-in capital	1,299,907,000	0
Retained earnings	226,837,000	0
Treasury stock, at cost; 928 shares at September 30, 2013	(9,811,000)	0
Accumulated other comprehensive loss	(5,147,000)	0
Total equity	1,513,021,000	654,672,000
Total liabilities, redeemable preferred member's equity and equity	13,293,295,000	1,687,536,000
Common Class A [Member]
Equity
Shares authorized: 1,000,000 at September 30, 2013; Shares issued: 123,515 at September 30, 2013; Shares outstanding: 122,587 at September 30, 2013	$ 1,235,000	$ 0