Pension plans - Additional tabular disclosures (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Applicable to pension benefit obligations:
Discount rate	3.30%	3.80%	4.10%
Expected long-term rate of return on plan assets	6.40%	6.10%	6.50%
Rate of compensation increase	2.80%	3.00%	3.40%
Discount rate applicable to net periodic pension expense	3.90%	4.20%	3.80%
Amounts recognized in other comprehensive income:
Accumulated other comprehensive income (loss), beginning of year	$ (839)	$ (1,193)
Amount included in net periodic pension expense	155	104
Actuarial gains and other	70	250
Accumulated other comprehensive income (loss), end of year	$ (614)	$ (839)	$ (1,193)