Segment Information - Schedule of consolidated non-current assets by geographic area (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 13,051	$ 13,805
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	7,072	7,793
Europe, Middle East and Africa [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	2,270	2,179
Asia Pacific [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	570	640
Americas other than U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	416	493
United Kingdom [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 2,723	$ 2,700