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, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		$ 456	$ 95
Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	12
Impaired commercial loans [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[2]	50	37
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure		424	37
Level 3 [Member] | Residential mortgages [Member]
Fair Value, Assets and Liabilities Measured on a Nonrecurring Basis [Line Items]
Assets, Fair Value Disclosure	[1]	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]	$ 50	$ 37
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		0.09	0.13
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		1.00	1.00
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]	0.45

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

[2] Certain commercial loans are considered impaired. As a matter of practical expedient, 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.