Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Dec. 31, 2019
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ 754		$ (120)		$ (114)		$ (519)
Other comprehensive income for period:
Net unrealized gains arising during period, net of tax		84		117		990		532
Reclassification adjustment for gains realized in net income (loss), net of tax		(39)	[1]	(35)	[1]	(84)	[2]	(58)	[2]
Reversal of provision for credit losses realized in net income (loss), net of tax		(1)	[1]	0	[1]	(1)		0
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income (loss), net of tax	[3]	12		4		19		11
Total other comprehensive income for period		56		86		924		485
Balance at end of period		810		(34)		810		(34)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period		137		197		(9)		301
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(40)		(3)		106		(107)
Total other comprehensive income (loss) for period		(40)		(3)		106		(107)
Balance at end of period		97		194		97		194
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(58)		(140)		(151)		(159)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(10)		(17)		75		(12)
Reclassification adjustment for losses realized in net income (loss), net of tax	[4]	0		7		8		21
Total other comprehensive income for period		(10)		(10)		83		9
Balance at end of period		(68)		(150)		(68)		(150)
Pension and postretirement benefit liability:
Balance at beginning of period		(3)		8		(3)		11
Change in unfunded pension and postretirement liability, net of tax		(2)		(1)		(2)		(4)
Total other comprehensive loss for period		(2)		(1)		(2)		(4)
Balance at end of period		(5)		7		(5)		7
Total accumulated other comprehensive income at end of period		$ 834		$ 17		834		17		$ (279)
As Previously Reported [Member]
Unrealized gains (losses) on investment securities:
Balance at beginning of period						(116)		(519)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Unrealized gains (losses) on investment securities:
Balance at beginning of period	[5]					$ 2		$ 0

[1] Changes in the allowance for credit losses on securities available-for-sale are included in the provision for credit losses in our consolidated statement of income (loss).

[2] Amount reclassified to net income (loss) is included in other securities gains, net in our consolidated statement of income (loss).

[3] Amount amortized to net income (loss) is included in interest income in our consolidated statement of income (loss). During 2014, we transferred securities from available-for-sale to held-to-maturity. At the date of transfer, AOCI included net pretax unrealized losses related to the transferred securities which are being amortized over the remaining contractual life of each security as an adjustment of yield in a manner consistent with the amortization of any premium or discount.

[4] Amount reclassified to net income (loss) is included in net interest income in our consolidated statement of income (loss).

[5] See Note 21, "New Accounting Pronouncements," for additional discussion.