Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 185	$ 160	$ 154
Charged to Costs and Expenses	73	57	5
Acquisition Impact	0	0	24
Charged to Other Accounts / Other	0	0	0
Deductions (1)	(101)	(32)	(23)
Balance at End of Period	157	185	160
Deferred tax valuation allowance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	150	286	308
Charged to Costs and Expenses	(64)	(121)	(95)
Acquisition Impact	0	0	0
Charged to Other Accounts / Other	0	0	0
Deductions (1)	(92)	15	(73)
Balance at End of Period	$ 178	$ 150	$ 286