Income taxes - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Statutory U.S. Corporate income tax rate			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
Unrecognized tax benefits	554		554	$ 485
Unrecognized tax benefits that would impact effective tax rate	$ 445		$ 445
Scenario, Forecast [Member]
Income Taxes [Line Items]
Statutory U.S. Corporate income tax rate		21.00%