Basis of presentation and significant accounting policies and practices - Additional Information (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Advertising expense	$ 46	$ 45	$ 46
Antidilutive securities excluded from computation of earnings per share, Amount (in shares)	12	11	0
Maximum length to maturity of a security, from the investment date, where it is classified as cash and cash equivalent	90 days
Length to maturity from the investment date of a security where the length of time is too short to be classified as a short-term investment	90 days
Consigned inventory	$ 275	$ 258