Property, plant and equipment - Depreciation expense (Detail) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Property, Plant and Equipment [Line Items] | |||
| Depreciation expense | $ 7,284 | $ 6,968 | $ 6,656 |
| Insurance and Other [Member] | |||
| Property, Plant and Equipment [Line Items] | |||
| Depreciation expense | 2,186 | 2,116 | 2,017 |
| Railroad, Utilities and Energy [Member] | |||
| Property, Plant and Equipment [Line Items] | |||
| Depreciation expense | $ 5,098 | $ 4,852 | $ 4,639 |