Income taxes - Narrative (Detail) - USD ($) $ in Millions |
3 Months Ended | 12 Months Ended | |
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Income Tax Disclosure [Abstract] | |||
| Statutory U.S. Corporate income tax rate | 21.00% | 35.00% | |
| Reduction in deferred income tax liabilities related to change in U.S. statutory income tax rate | $ 35,600 | ||
| Income tax benefit related to change in U.S. statutory income tax rate | 29,600 | ||
| Increase in regulatory liabilities for deferred income tax liability reduction related to change in U.S. statutory income tax rate | 6,000 | ||
| Income tax charge with respect to deemed repatriation of accumulated undistributed earnings of foreign subsidiaries | 1,400 | ||
| Net income tax benefit as a result of enactment of the TCJA | $ 28,200 | 28,200 | |
| Reduction of estimate of income taxes on deemed repatriation of earnings of foreign subsidiaries | $ 141 | ||
| Unrecognized tax benefits | $ 554 | 549 | $ 554 |
| Unrecognized tax benefits that would impact effective tax rate | $ 452 | ||