Segment Information - Schedule of consolidated revenue by geographic area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 3,288	$ 2,742	$ 2,805	$ 2,847	$ 3,299	$ 2,880	$ 2,919	$ 2,947	$ 11,682	$ 12,045	$ 11,815
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									6,063	5,824	5,574
Americas other than U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,053	1,176	1,214
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,527	1,623	1,544
Europe, Middle East and Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,909	2,189	2,304
Asia Pacific [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 1,130	$ 1,233	$ 1,179