Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2020		Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		$ 40,672			$ 35,663
Securities available-for-sale, unrealized losses		(72)			(330)
Securities available-for-sale, unrealized gains		1,109			261
Securities available-for-sale, allowance for credit losses	[2]	(1)	[1]	$ (3)
Securities available-for-sale, amortized cost		39,636
Securities available-for-sale, amortized cost					35,732
Securities held-to-maturity, fair value		9,369			13,431
Securities held-to-maturity, unrealized losses		0			(27)
Securities held-to-maturity, unrealized gain		388			165
Securities held-to-maturity, allowance for credit losses	[2]	(2)	[1]	$ (2)
Securities held-to-maturity, amortized cost		8,983
Securities held-to-maturity, amortized cost					13,293
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		16,648			16,078
Securities available-for-sale, unrealized losses		(47)			(269)
Securities available-for-sale, unrealized gains		608			128
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		16,087
Securities available-for-sale, amortized cost					16,219
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		6,264			3,402
Securities available-for-sale, unrealized losses		(2)			(13)
Securities available-for-sale, unrealized gains		280			57
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		5,986
Securities available-for-sale, amortized cost					3,358
Securities held-to-maturity, fair value		1,181			1,653
Securities held-to-maturity, unrealized losses		0			(1)
Securities held-to-maturity, unrealized gain		44			22
Securities held-to-maturity, allowance for credit losses	[1]	0
Securities held-to-maturity, amortized cost		1,137
Securities held-to-maturity, amortized cost					1,632
Collateralized Mortgage Obligations, Issued by U.S. Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		1,693			347
Securities available-for-sale, unrealized losses		(2)			(1)
Securities available-for-sale, unrealized gains		19			3
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		1,676
Securities available-for-sale, amortized cost					345
Securities held-to-maturity, fair value		833			1,454
Securities held-to-maturity, unrealized losses		0			(4)
Securities held-to-maturity, unrealized gain		53			40
Securities held-to-maturity, allowance for credit losses	[1]	0
Securities held-to-maturity, amortized cost		780
Securities held-to-maturity, amortized cost					1,418
Direct Agency Obligations U S Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		1,253			1,403
Securities available-for-sale, unrealized losses		0			0
Securities available-for-sale, unrealized gains		17			21
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		1,236
Securities available-for-sale, amortized cost					1,382
Mortgage-backed Securities, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		7,145			9,997
Securities available-for-sale, unrealized losses		0			(41)
Securities available-for-sale, unrealized gains		152			29
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		6,993
Securities available-for-sale, amortized cost					10,009
Securities held-to-maturity, fair value		2,023			3,021
Securities held-to-maturity, unrealized losses		0			0
Securities held-to-maturity, unrealized gain		76			17
Securities held-to-maturity, allowance for credit losses	[1]	0
Securities held-to-maturity, amortized cost		1,947
Securities held-to-maturity, amortized cost					3,004
Collateralized Mortgage Obligations, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		2,104			747
Securities available-for-sale, unrealized losses		(11)			(4)
Securities available-for-sale, unrealized gains		22			10
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		2,093
Securities available-for-sale, amortized cost					741
Securities held-to-maturity, fair value		5,319			7,290
Securities held-to-maturity, unrealized losses		0			(22)
Securities held-to-maturity, unrealized gain		212			85
Securities held-to-maturity, allowance for credit losses	[1]	0
Securities held-to-maturity, amortized cost		5,107
Securities held-to-maturity, amortized cost					7,227
Direct Agency Obligations, Issued or Guaranteed by US Government Agency [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		301			260
Securities available-for-sale, unrealized losses		0			0
Securities available-for-sale, unrealized gains		5			6
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		296
Securities available-for-sale, amortized cost					254
Obligations of U.S. states and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Securities held-to-maturity, fair value		11			11
Securities held-to-maturity, unrealized losses		0			0
Securities held-to-maturity, unrealized gain		1			1
Securities held-to-maturity, allowance for credit losses	[1]	0
Securities held-to-maturity, amortized cost		10
Securities held-to-maturity, amortized cost					10
Residential mortgage asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities held-to-maturity, fair value		2			2
Securities held-to-maturity, unrealized losses		0			0
Securities held-to-maturity, unrealized gain		2			0
Securities held-to-maturity, allowance for credit losses	[1]	(2)
Securities held-to-maturity, amortized cost		2
Securities held-to-maturity, amortized cost					2
Home Equity Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		27			32
Securities available-for-sale, unrealized losses		0			(2)
Securities available-for-sale, unrealized gains		0			0
Securities available-for-sale, allowance for credit losses	[1]	(1)
Securities available-for-sale, amortized cost		28
Securities available-for-sale, amortized cost					34
Other securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value		104			111
Securities available-for-sale, unrealized losses		(10)			0
Securities available-for-sale, unrealized gains		0			3
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost		114
Securities available-for-sale, amortized cost					108
Foreign debt securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale, fair value	[3]	5,133			3,286
Securities available-for-sale, unrealized losses	[3]	0			0
Securities available-for-sale, unrealized gains	[3]	6			4
Securities available-for-sale, allowance for credit losses	[1]	0
Securities available-for-sale, amortized cost	[3]	$ 5,127
Securities available-for-sale, amortized cost	[3]				$ 3,282

[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.

[3] Foreign debt securities represent public sector entity, bank or corporate debt.