Income taxes - Liabilities (Detail) - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Income Tax Disclosure [Abstract] | ||
| Currently payable (receivable) | $ 323 | $ (129) |
| Deferred | 50,503 | 56,182 |
| Other income tax liabilities | 549 | 554 |
| Income taxes, principally deferred | $ 51,375 | $ 56,607 |