Acquisitions (Acquired Finite-Lived Intangible Assets) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 73
Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 72
Finite-lived intangible assets, remaining amortization period	9 years
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 1
Finite-lived intangible assets, remaining amortization period	4 years