Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 595	$ 656	$ 1,939	$ 1,854
Depreciation and amortization	(995)	(1,048)	(2,958)	(3,136)
Other unassigned operating expenses	(796)	(857)	(2,439)	(2,571)
Interest expense and other (expense) income, net	(346)	(327)	(993)	(968)
Income tax expense	(97)	(124)	(362)	(346)
Net income	152	205	584	540
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	1,878	1,980	5,870	6,124
Segment reconciling items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues	304	409	919	924
Depreciation and amortization	(995)	(1,048)	(2,958)	(3,136)
Other unassigned operating expenses	(592)	(685)	(1,892)	(2,058)
Interest expense and other (expense) income, net	(346)	(327)	(993)	(968)
Income tax expense	$ (97)	$ (124)	$ (362)	$ (346)