Segment Information - Schedule of consolidated revenue by geographic area (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 3,299	$ 2,880	$ 2,919	$ 2,947	$ 3,209	$ 2,794	$ 2,897	$ 2,915	$ 12,045	$ 11,815	$ 11,514
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									5,824	5,574	5,336
Americas other than U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,176	1,214	1,190
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									1,623	1,544	1,541
Europe, Middle East and Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									2,189	2,304	2,271
Asia Pacific [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 1,233	$ 1,179	$ 1,176