Segment Information - Foreign and U.S. Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 36,666	$ 33,594	$ 31,003
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	33,185	30,070	27,868
Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 3,481	$ 3,524	$ 3,135