Investments - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended						12 Months Ended
	Dec. 29, 2015	Dec. 31, 2016	Nov. 30, 2016	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 13, 2015	Mar. 10, 2015
Schedule of Equity Method Investments [Line Items]
Dividend income										$ 59,295	$ 63,971	$ 45,910
Total assets held at fair value		$ 2,614,729		$ 2,614,729						2,614,729	2,517,735
Investment income and other, net										$ 393,365	$ 421,384	41,232
Warrants received (in shares)		6,182,000		6,182,000						6,182,000	21,605,000
Investments held at fair value		$ 9,198		$ 9,198						$ 9,198	$ 1,814
Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Income from equity method investments										11,500	21,000	36,000
Gains on transfers to trading											500
Realized gains on cost-method investments											9,300
Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Dividend income										$ 2,300
Aperture Holdings, LP | Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Pre-tax gain from sale of investment	$ 10,500										10,500
BATS
Schedule of Equity Method Investments [Line Items]
Number of shares sold			13,000,000
Amount of equity method investment sold						$ 46,400
Ownership percentage		0.50%
Remaining shares owned		206,000		206,000						206,000
Investments held at fair value		$ 6,900		$ 6,900						$ 6,900
BATS | Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Pre-tax gain from sale of investment										364,400
Financial instruments owned, at fair value
Schedule of Equity Method Investments [Line Items]
Amount reclassified from available-for-sale securities to trading							$ 2,800
Other Assets
Schedule of Equity Method Investments [Line Items]
Total assets held at fair value											$ 2,800
BATS Global Markets and Direct Edge Holdings LLC
Schedule of Equity Method Investments [Line Items]
Merger related transaction costs									$ 6,600
BATS Global Markets and Direct Edge Holdings LLC | Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Investment income and other, net									9,600
Direct Edge Holdings LLC
Schedule of Equity Method Investments [Line Items]
Investment income and other, net									$ 16,200
BATS
Schedule of Equity Method Investments [Line Items]
Ownership percentage											16.70%
Percentage of voting interest held											19.90%
tradeMonster Group, Inc. | Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Investment income and other, net								$ 15,100				$ 15,100
TM Holdings
Schedule of Equity Method Investments [Line Items]
Investment income and other, net								$ 17,600
IPO | BATS
Schedule of Equity Method Investments [Line Items]
Number of shares sold						2,600,000.0
IPO | BATS | Investment income and other, net
Schedule of Equity Method Investments [Line Items]
Pre-tax gain from sale of investment										33,400
Open Market Transactions | BATS
Schedule of Equity Method Investments [Line Items]
Number of shares sold				2,000,000
Block Sale | BATS
Schedule of Equity Method Investments [Line Items]
Number of shares sold				2,200,000.0
Swap Transaction
Schedule of Equity Method Investments [Line Items]
Pre-tax gain from sale of investment										$ 331,000
Warrants received (in shares)			8,100,000
Warrants retained (in shares)		1,100,000	1,100,000	1,100,000						1,100,000
Portion of transaction costs (in shares)			46,000
Swap Transaction | BATS
Schedule of Equity Method Investments [Line Items]
Number of shares exchanged in transaction			8,900,000
Number of shares retained in transaction		94,000		94,000						94,000
Number of shares treated as derivatives		48,000		48,000						48,000
Receivable included in other assets		$ 2,900		$ 2,900						$ 2,900
Liability included in accrued expenses and other liabilities		$ 2,900		$ 2,900						$ 2,900
Fair value of portion of transaction costs			$ 1,400
General Atlantic | Common Class A | Swap Transaction
Schedule of Equity Method Investments [Line Items]
Number of shares exchanged in transaction			18,700,000	18,700,000
Senior Secured Notes | 6.875% Senior Secured Notes
Schedule of Equity Method Investments [Line Items]
Interest rate (percent)		6.875%		6.875%		6.875%				6.875%	6.875%		6.875%	6.875%
Jefferies LLC | Common Class A | Swap Transaction
Schedule of Equity Method Investments [Line Items]
Percentage of ownership before transaction					19.00%
Ownership percentage				24.00%
Jefferies LLC | Swap Transaction
Schedule of Equity Method Investments [Line Items]
Payments of stock issuance costs			$ 2,900