Summary of Significant Accounting Policies - Accounting standards (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for expected credit losses	$ 393	$ 289
Total shareholders’ equity	30,217		$ 25,998	$ 21,312
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total shareholders’ equity	$ 52,767		48,074	44,033	$ 41,579
Retained income | Cumulative effect of change in accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total shareholders’ equity			$ (88)	$ 21	$ 1,088
Retained income | Accounting Standards Update 2016-13 | Cumulative effect of change in accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total shareholders’ equity		$ 88