Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
|
| Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] | |||
| Balance, January 1 | $ 84 | $ 108 | $ 91 |
| Additions based on tax positions related to the current year | 4 | 11 | 10 |
| Additions for tax positions of prior years | 189 | 3 | 52 |
| Reductions for tax positions of prior years | (2) | (21) | (9) |
| Settlements with tax authorities | (32) | (17) | (36) |
| Balance, December 31 | 243 | 84 | 108 |
| Interest income (expense) recognized in the year ended December 31 | 4 | 8 | 6 |
| Interest receivable (payable) as of December 31 | $ 13 | $ 9 | $ 0 |