Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (570)	$ (244)	$ (299)	$ (461)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax		(143)	20	(409)	244
Reclassification adjustment for (gains) losses realized in net income, net of tax	[1]	3	(3)	(8)	(18)
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	5	9	13
Total other comprehensive income (loss) for period		(137)	22	(408)	239
Balance at end of period		(707)	(222)	(707)	(222)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period		118	86	(23)	174
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		(47)	(45)	94	(133)
Total other comprehensive income (loss) for period		(47)	(45)	94	(133)
Balance at end of period		71	41	71	41
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(187)	(159)	(199)	(157)
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax		18	(2)	23	(7)
Reclassification adjustment for (gains) losses realized in net income, net of tax	[3]	4	4	11	7
Total other comprehensive income (loss) for period		22	2	34	0
Balance at end of period		(165)	(157)	(165)	(157)
Pension and postretirement benefit liability:
Balance at beginning of period		3	0	3	0
Change in unfunded pension and postretirement liability, net of tax				0	0
Total other comprehensive income for period				0	0
Balance at end of period		3	0	3	0
Total accumulated other comprehensive loss at end of period		$ (798)	$ (338)	(798)	(338)	$ (431)
AOCI included in pretax unrealized losses								$ 234
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	[4]					4	$ 0
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	[4]					(53)	0
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	[4]					(4)	0
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	[4]					(35)	0
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	[4]					1	0
Accounting Standards Update 2016-01, Fair Value Option [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	[5]					$ 0	$ 174
As Previously Reported [Member]
Unrealized gains (losses) on investment securities:
Balance at beginning of period				(250)	(461)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period				(19)	0
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period				(164)	(157)
Pension and postretirement benefit liability:
Balance at beginning of period				$ 2	$ 0

[1] Amount reclassified to net income is included in other securities gains (losses), 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 of $234 million 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 interest income (expense) in our consolidated statement of income.

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

[5] See Note 2, "Summary of Significant Accounting Polices and New Accounting Pronouncements," in our 2017 Form 10-K for additional discussion.