Securities - Additional Information (Detail) - USD ($)		3 Months Ended
		Jun. 30, 2020	Sep. 30, 2020		Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, allowance for credit losses	[2]		$ 2,000,000	[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
Equity securities without readily determinable fair values, impairment loss						$ 0
Other assets [Member]
Debt Securities, Available-for-sale [Line Items]
Equity securities carried at fair value			283,000,000			283,000,000
Equity securities without readily determinable fair values			14,000,000			12,000,000
Federal Home Loan Bank Stock			304,000,000			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 21, "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 4, "Securities," for additional information regarding the allowance for credit losses associated with our security portfolios.