Unsecured Borrowings - Subordinated Long-Term Borrowings (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total subordinated Long-term Borrowings, par amount	$ 14,987	$ 15,017
Total subordinated Long-term Borrowings	$ 20,464	$ 18,308
Effective weighted average interest rate on long-term subordinated borrowings, after hedging	1.80%	3.42%
Subordinated Debt [Member]
Debt Instrument [Line Items]
Long-term debt, par amount	$ 14,019	$ 14,041
Long-term subordinated debt outstanding	$ 18,987	$ 16,980
Effective weighted average interest rate of long-term subordinated debt, after hedging	1.83%	3.46%
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Long-term debt, par amount	$ 968	$ 976
Long-term junior subordinated debt	$ 1,477	$ 1,328
Effective weighted average interest rate of long-term junior subordinated debt, after hedging	1.39%	2.85%