Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 751	$ 656	$ 549	$ 649	$ 483	$ 619	$ 655	$ 653	$ 2,605	$ 2,410	$ 1,453
Depreciation and amortization									(4,189)	(4,428)	(4,541)
Impairment of goodwill									0	0	(1,092)
Other unassigned operating expenses									(3,328)	(3,347)	(3,502)
Income tax expense									(438)	(338)	(463)
Other expenses, net									(1,289)	(1,300)	(1,229)
Net income (loss)	$ 338	$ 205	$ 143	$ 192	$ 188	$ 188	$ 193	$ 203	878	772	(239)
Operating segments (segments)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income									8,209	8,519	8,928
Unallocated amount to segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues									1,232	1,003	1,000
Depreciation and amortization									(4,189)	(4,428)	(4,541)
Impairment of goodwill									0	0	(1,092)
Other unassigned operating expenses									(2,647)	(2,684)	(2,842)
Income tax expense									(438)	(338)	(463)
Other expenses, net									$ (1,289)	$ (1,300)	$ (1,229)