Description of Business and Accounting Policies - Revenue (Details) - Allowance for Sales Returns - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance [Line Items]
Allowance for returns	$ 62	$ 156	$ 153
Additions to allowance for returns	1,800	1,500	1,300
Deductions to allowance for returns	$ 1,900	$ 1,500	$ 1,300