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 | |