New accounting pronouncements - Narrative (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	Jan. 01, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 484,431	$ 444,626
ASU 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
ASU 2016-13 [Member] | Cumulative effect, period of adoption, adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			$ (388)