Pension plans - Defined benefit pension plans - Narrative (Detail) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 18,824	$ 17,750	$ 15,259
Amounts recognized in the Consolidated Balance Sheets:
Funded status reflected in assets	1,176	644
Funded status reflected in liabilities	2,986	2,997
Estimated future benefit payments
2018	1,058
2019	1,004
2020	1,024
2021	1,019
2022	1,019
2023 - 2027	5,095
Expected contribution to pension plans during the next year	251
Unfunded Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 1,300	$ 1,200