Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (120)	$ (570)	$ (519)	$ (299)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		117	(143)	532	(409)
Reclassification adjustment for (gains) losses realized in net income (loss), net of tax	[1]	(35)	3	(58)	(8)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income (loss), net of tax	[2]	4	3	11	9
Total other comprehensive income (loss) for period		86	(137)	485	(408)
Balance at end of period		(34)	(707)	(34)	(707)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period		197	118	301	(23)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(3)	(47)	(107)	94
Total other comprehensive income (loss) for period		(3)	(47)	(107)	94
Balance at end of period		194	71	194	71
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(140)		(159)
Balance at beginning of period			(187)		(199)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(17)		(12)
Reclassification adjustment for losses realized in net income (loss), net of tax	[3]	7		21
Total other comprehensive income for period		(10)		9
Net unrealized gains (losses) arising during period, net of tax			18		23
Reclassification adjustment for gains (losses) realized in net income (loss), net of tax	[3]		4		11
Total other comprehensive income (loss) for period			22		34
Balance at end of period			(165)		(165)
Balance at end of period		(150)		(150)
Pension and postretirement benefit liability:
Balance at beginning of period		8	3	11	3
Change in unfunded pension and postretirement liability, net of tax		(1)	0	(4)	0
Total other comprehensive loss for period		(1)	0	(4)	0
Balance at end of period		7	3	7	3
Total accumulated other comprehensive income (loss) at end of period		$ 17	$ (798)	$ 17	(798)	$ (366)
Accounting Standards Update 2016-01, Equity Investments Previously Classified as Available-for-sale [Member] | Unrealized Gains (Losses) on Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption						0	$ 4
Accounting Standards Update 2018-02 [Member] | Unrealized Gains (Losses) on Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption						0	(53)
Accounting Standards Update 2018-02 [Member] | Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Our Own Credit Spread [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption						0	(4)
Accounting Standards Update 2018-02 [Member] | Unrealized Gains (Losses) on Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption						0	(35)
Accounting Standards Update 2018-02 [Member] | Pension and Postretirement Benefit Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of new accounting principle in period of adoption						$ 0	$ 1
As Previously Reported [Member]
Unrealized gains (losses) on investment securities:
Balance at beginning of period					(250)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period					(19)
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period					(164)
Pension and postretirement benefit liability:
Balance at beginning of period					$ 2

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

[2] 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.

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