Segment Information (Details 3) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income	$ 524	$ 487	$ 367	$ 631	$ 405	$ 593	$ 647	$ 688	$ 2,009	$ 2,333	$ 2,579
Depreciation and amortization									(3,936)	(3,916)	(4,189)
Non-segment expenses									(3,508)	(3,447)	(3,354)
Other expenses, net									(1,469)	(1,313)	(1,263)
INCOME BEFORE INCOME TAX EXPENSE									540	1,020	1,316
Income tax benefit (expense)									849	(394)	(438)
Net income	$ 1,117	$ 92	$ 17	$ 163	$ 42	$ 152	$ 196	$ 236	1,389	626	878
Operating segments (segments)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment income									7,534	7,685	8,146
Unallocated amount to segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Other operating revenues									732	704	729
Depreciation and amortization									(3,936)	(3,916)	(4,189)
Non-segment expenses									(2,321)	(2,140)	(2,107)
Other expenses, net									(1,469)	(1,313)	(1,263)
INCOME BEFORE INCOME TAX EXPENSE									540	1,020	1,316
Income tax benefit (expense)									$ 849	$ (394)	$ (438)