Leases - Summary of remaining operating lease payments (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Operating Lease Liabilities, Payments Due [Abstract]
2019	$ 933
2019		$ 1,310
2020	1,312	1,268
2021	1,091	1,048
2022	859	820
2023	685	658
Thereafter	2,183	2,079
Total lease payments	7,063	7,183
Total lease payments	7,063	$ 7,183
Amount representing interest	(1,053)
Lease liabilities	$ 6,010