Commitments, Contingencies and Guarantees (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Line Items]
Financial guarantees obtained	$ 13,000,000	$ 13,000,000
Margin deposits contributed to Cantor Fitzgerald	$ 19,000,000
eSpeed
Commitments and Contingencies Disclosure [Line Items]
Contingent future issuance of common stock (in shares)	992,247
Revenue required to trigger annual issuance of Nasdaq common stock	$ 25,000,000
Property Lease Guarantee
Commitments and Contingencies Disclosure [Line Items]
Other guarantees	3,000,000	3,000,000
Escrow Agreement
Commitments and Contingencies Disclosure [Line Items]
Contingency, accrual	13,000,000
Escrow Agreement | Other current liabilities
Commitments and Contingencies Disclosure [Line Items]
Contingency, accrual	9,000,000
Escrow Agreement | Other non-current liabilities
Commitments and Contingencies Disclosure [Line Items]
Contingency, accrual	4,000,000
Clearinghouse Credit Facilities
Commitments and Contingencies Disclosure [Line Items]
Credit facility, available liquidity	$ 188,000,000	$ 170,000,000