Acquisitions and Divestitures - Components of Business Held For Sale (Details) - Disposal Group, Held-for-sale, Not Discontinued Operations - QuantHouse $ in Millions	Dec. 31, 2017 USD ($)
Components of Assets and Liabilities Held For Sale
Accounts receivable, net	$ 4
Other assets	3
Assets of a business held for sale	7
Accounts payable and accrued expenses	3
Unearned revenue	7
Other liabilities	35
Liabilities of a business held for sale	$ 45