Segment and Geographic Area Reporting - Revenue and Long-lived Assets by Geography (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) service_offering	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | service_offering	1
Total Revenues	$ 9,066	$ 9,662	$ 9,991
Long-Lived Assets	1,231	1,094
United States
Segment Reporting Information [Line Items]
Total Revenues	5,429	5,610	5,790
Long-Lived Assets	769	670
Europe
Segment Reporting Information [Line Items]
Total Revenues	2,326	2,625	2,697
Long-Lived Assets	305	277
Other areas
Segment Reporting Information [Line Items]
Total Revenues	1,311	1,427	$ 1,504
Long-Lived Assets	$ 157	$ 147