Summary of Significant Accounting Policies (Summary of Activity of Allowance For Doubtful Accounts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts
Beginning balance	$ 150	$ 169	$ 209
Bad debt expense	48	51	48
Write-offs	(70)	(71)	(86)
Translation and other adjustments	15	1	(2)
Ending balance	$ 143	$ 150	$ 169