Securities - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended
		Jun. 30, 2020	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, allowance for credit losses	[2]		$ 1,000,000	[1]			$ 3,000,000
Securities held-to-maturity, allowance for credit losses	[2]		2,000,000	[1]			$ 2,000,000
Securities held-to-maturity, past due			0
Securities held-to-maturity, nonaccrual			0
Equity securities without readily determinable fair values, impairment loss adjustment			2,000,000		$ 0	$ 0
Other assets [Member]
Debt Securities, Available-for-sale [Line Items]
Equity securities carried at fair value			284,000,000		283,000,000
Equity securities without readily determinable fair values			14,000,000		12,000,000
Federal Home Loan Bank Stock			259,000,000		110,000,000
Federal Reserve Bank Stock			$ 559,000,000		$ 559,000,000
Accounting Standards Update 2020-04 [Member]
Debt Securities, Available-for-sale [Line Items]
Carrying value of LIBOR-linked variable rate held-to-maturity secrurities maturing beyond 2021, sold in the period		$ 340,000,000
Gain on sale of LIBOR-linked variable rate held-to-maturity securities maturing beyond 2021 (less than)		$ 1,000,000

[1] As discussed in Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," beginning January 1, 2020, an allowance for credit losses is recognized for debt securities while, prior to January 1, 2020, debt securities were assessed for other-than-temporary impairment. At December 31, 2019, we did not consider any of our debt securities to be other-than-temporarily impaired.

[2] See Note 5, "Securities," for additional information regarding the allowance for credit losses associated with our security portfolios.