Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 656	$ 619	$ 1,854	$ 1,927
Depreciation and amortization	(1,048)	(1,097)	(3,136)	(3,297)
Other unassigned operating expenses	(857)	(823)	(2,571)	(2,497)
Other income (expense), net	(327)	(320)	(968)	(974)
Income tax expense	(124)	(111)	(346)	(369)
Net income	205	188	540	584
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	1,982	2,103	6,122	6,462
Segment reconciling items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues	409	250	924	757
Depreciation and amortization	(1,048)	(1,097)	(3,136)	(3,297)
Other unassigned operating expenses	(687)	(637)	(2,056)	(1,995)
Other income (expense), net	(327)	(320)	(968)	(974)
Income tax expense	$ (124)	$ (111)	$ (346)	$ (369)