Additional Information (Schedule of Valuation and Qualifying Accounts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance For Discounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	$ 0	$ 0	$ 0
Charged to costs and expenses	626	628	618
Deductions	(626)	(628)	(618)
Balance at end of year	0	0	0
Allowance For Returned Goods [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	49	68	46
Charged to costs and expenses	130	133	217
Deductions	(139)	(152)	(195)
Balance at end of year	$ 40	$ 49	$ 68