Segment Information - Schedule of consolidated revenue by geographic area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 3,262	$ 2,760	$ 2,800	$ 2,805	$ 3,227	$ 2,752	$ 2,836	$ 2,867	$ 11,627	$ 11,682	$ 12,045
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									6,078	6,063	5,824
Americas other than U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									995	1,053	1,176
United Kingdom
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,378	1,527	1,623
Europe, Middle East and Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,760	1,909	2,189
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 1,416	$ 1,130	$ 1,233