Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 650	$ 549	$ 1,344	$ 1,198
Depreciation and amortization	(987)	(1,048)	(1,963)	(2,088)
Other unassigned operating expenses	(812)	(863)	(1,643)	(1,714)
Interest expense and other income, net	(333)	(315)	(647)	(641)
Income tax expense	(121)	(91)	(265)	(222)
Net income	196	143	432	335
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	1,950	2,021	3,992	4,144
Segment reconciling items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues	307	258	615	515
Depreciation and amortization	(987)	(1,048)	(1,963)	(2,088)
Other unassigned operating expenses	(620)	(682)	(1,300)	(1,373)
Interest expense and other income, net	(333)	(315)	(647)	(641)
Income tax expense	$ (121)	$ (91)	$ (265)	$ (222)