Pension plans - Additional tabular disclosures (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Discount rate applicable to pension benefit obligations	2.30%	3.10%	3.90%
Expected long-term rate of return on plan assets	6.20%	6.40%	6.40%
Rate of compensation increase	2.60%	2.50%	2.60%
Discount rate applicable to net periodic pension expense	3.10%	4.00%	3.40%
Amounts recognized in other comprehensive income:
Accumulated other comprehensive income (loss), beginning of year	$ (1,896)	$ (1,184)
Amount included in net periodic pension expense	141	94
Actuarial gains (losses) and other	(496)	(806)
Accumulated other comprehensive income (loss), end of year	$ (2,251)	$ (1,896)	$ (1,184)