Income taxes - Deferred income tax assets and liabilities (Details) - USD ($) $ in Millions |
Dec. 31, 2015 |
Dec. 31, 2014 |
|---|---|---|
| Income Tax [Line Items] | ||
| Current deferred income tax assets | $ 0 | |
| Noncurrent deferred income tax assets | $ 201 | 180 |
| Current deferred income tax liabilities | 0 | |
| Noncurrent deferred income tax liabilities | (37) | (64) |
| Net deferred income tax asset | $ 164 | 116 |
| 2014 As Reported | ||
| Income Tax [Line Items] | ||
| Current deferred income tax assets | 347 | |
| Noncurrent deferred income tax assets | 172 | |
| Current deferred income tax liabilities | (4) | |
| Noncurrent deferred income tax liabilities | (399) | |
| Net deferred income tax asset | 116 | |
| Effect of Applying New Standard | ||
| Income Tax [Line Items] | ||
| Current deferred income tax assets | (347) | |
| Noncurrent deferred income tax assets | 8 | |
| Current deferred income tax liabilities | 4 | |
| Noncurrent deferred income tax liabilities | 335 | |
| Net deferred income tax asset | $ 0 |