Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total interest-bearing liabilities, fair value	$ 84,421	$ 89,815
Difference [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total interest-bearing liabilities, fair value	1,794	383
Senior Unsecured Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt principal amount	9,500
Senior Unsecured Debt [Member] | Difference [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total interest-bearing liabilities, fair value	$ 975	$ 56