Fair Value Option - Gains and Losses on Other Financial Assets and Financial Liabilities at Fair Value (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Option Gains/(Losses)	$ (701)	$ (1,374)	$ 311	$ (8,368)
Unsecured Short-Term Borrowings Including Current Portion of Unsecured Long-Term Borrowings at Fair Value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Option Gains/(Losses)	(406)	(293)	1,723	(2,378)
Unsecured Long-Term Borrowings at Fair Value [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Option Gains/(Losses)	(143)	(929)	(1,166)	(5,078)
Fair Value Option Other [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Option Gains/(Losses)	$ (152)	$ (152)	$ (246)	$ (912)