Acquisitions and Dispositions (Acquired Finite-Lived Intangible Assets) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 232
Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 215
Finite-lived intangible assets, remaining amortization period	11 years
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 17
Finite-lived intangible assets, remaining amortization period	4 years