Fair Value Measurements - Quantitative Information about Nonrecurring Fair Value Measurement of Assets and Liabilities Classified as Level 3 (Detail) - Fair Value, Nonrecurring [Member] $ in Millions		Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		$ 658	$ 456
Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	4	12
Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[2]	270	50
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		273	424
Level 3 [Member] | Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	0	0
Level 3 [Member] | Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[2]	$ 270	$ 50
Measurement Input, Loss Severity Rates [Member] | Level 3 [Member] | Impaired commercial loans [Member] | Minimum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans, Measurement Input		0	0.09
Measurement Input, Loss Severity Rates [Member] | Level 3 [Member] | Impaired commercial loans [Member] | Maximum [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans, Measurement Input		0.76	1
Measurement Input, Loss Severity Rates [Member] | Level 3 [Member] | Impaired commercial loans [Member] | Weighted Average [Member] | Third Party Appraisal [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Loans, Measurement Input	[3]	0.35	0.45

[1] At December 31, 2020 and 2019, the fair value of the loans held for sale was below cost.

[2] Certain commercial loans are individually assessed for impairment. We measure the credit impairment of a collateral-dependent loan based on the fair value of the collateral asset. The collateral often involves real estate properties that are illiquid due to market conditions. As a result, these loans are classified as a Level 3 fair value measurement within the fair value hierarchy.

[3] Weighted average is calculated based on the carrying value of the loans.