Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 694	$ 649
Depreciation and amortization	(976)	(1,040)
Other unassigned operating expenses	(831)	(851)
Other income (expense), net	(314)	(326)
Income tax expense	(144)	(131)
Net income	236	192
Operating segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	2,042	2,123
Segment reconciling items
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues	308	257
Depreciation and amortization	(976)	(1,040)
Other unassigned operating expenses	680	(691)
Other income (expense), net	(314)	(326)
Income tax expense	$ (144)	$ (131)