Acquisitions (Intangible Assets) (Details) - eVestment - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017
Finite Lived And Indefinite Lived Intangible Assets [Line Items]
Acquired Intangible Assets, Finite-lived	$ 405
Customer relationships
Finite Lived And Indefinite Lived Intangible Assets [Line Items]
Acquired Intangible Assets, Finite-lived	$ 378
Discount rate used	9.30%
Estimated average useful life	14 years
Amortization period of intangible assets for tax purposes	15 years
Trade name
Finite Lived And Indefinite Lived Intangible Assets [Line Items]
Acquired Intangible Assets, Finite-lived	$ 13
Discount rate used	9.20%
Estimated average useful life	8 years	8 years
Technology
Finite Lived And Indefinite Lived Intangible Assets [Line Items]
Acquired Intangible Assets, Finite-lived	$ 14
Discount rate used	9.20%
Estimated average useful life	8 years	8 years