Goodwill and other intangible assets - Goodwill (Detail) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Goodwill and Intangible Assets Disclosure [Abstract] | ||
| Balance at beginning of year | $ 81,258 | $ 79,486 |
| Acquisitions of businesses | 376 | 1,545 |
| Other, including foreign currency translation | (609) | 227 |
| Balance at end of year | $ 81,025 | $ 81,258 |