Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (340)	$ (494)	$ (519)	$ (299)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		235	(73)	415	(266)
Reclassification adjustment for (gains) losses realized in net income, net of tax	[1]	(18)	(7)	(23)	(11)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income, net of tax	[2]	3	4	7	6
Total other comprehensive income (loss) for period		220	(76)	399	(271)
Balance at end of period		(120)	(570)	(120)	(570)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period		190	(11)	301	(23)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		7	129	(104)	141
Total other comprehensive income (loss) for period		7	129	(104)	141
Balance at end of period		197	118	197	118
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(145)		(159)
Balance at beginning of period			(197)		(199)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(1)		5
Reclassification adjustment for losses realized in net income, net of tax	[3]	6		14
Total other comprehensive income for period		5		19
Net unrealized gains (losses) arising during period, net of tax			6		5
Reclassification adjustment for (gains) losses realized in net income, net of tax	[3]		4		7
Total other comprehensive income (loss) for period			10		12
Balance at end of period			(187)		(187)
Balance at end of period		(140)		(140)
Pension and postretirement benefit liability:
Balance at beginning of period		8	4	11	3
Change in unfunded pension and postretirement liability, net of tax		0	(1)	(3)	0
Total other comprehensive loss for period		0	(1)	(3)	0
Balance at end of period		8	3	8	3
Total accumulated other comprehensive loss at end of period		$ (55)	$ (636)	$ (55)	(636)	$ (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 is included in other securities gains, net in our consolidated statement of income.

[2] Amount amortized to net income is included in interest income in our consolidated statement of income. 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 is included in net interest income in our consolidated statement of income.