Accumulated Other Comprehensive Loss - (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Unrealized gains (losses) on investment securities:
Balance at beginning of period		$ (299)	$ (461)
Other comprehensive income for period:
Net unrealized gains (losses) arising during period, net of tax of $(63) million and $64 million, respectively		(193)	106
Reclassification adjustment for gains realized in net income (loss), net of tax of $(1) million and $(2) million, respectively(2)	[1]	(4)	(3)
Amortization of net unrealized losses on securities transferred from available-for-sale to held-to-maturity realized in net income (loss), net of tax of $1 million and $3 million, respectively(3)	[2]	2	4
Total other comprehensive income (loss) for period		(195)	107
Balance at end of period		(494)	(354)
Unrealized gains (losses) on fair value option liabilities attributable to our own credit spread:
Balance at beginning of period		(23)	174
Other comprehensive income (loss) for period:
Net unrealized gains (losses) arising during period, net of tax of $5 million and $(46) million, respectively		12	(79)
Total other comprehensive income (loss) for period		12	(79)
Balance at end of period		(11)	95
Unrealized gains (losses) on derivatives designated as cash flow hedges:
Balance at beginning of period		(199)	(157)
Other comprehensive income (loss) for period:
Net unrealized losses arising during period, net of tax is less then $(1) million and less than $(1) million, respectively		(1)	(1)
Reclassification adjustment for losses realized in net income (loss), net of tax of $1 million and $1 million, respectively(5)	[3]	3	2
Total other comprehensive income (loss) for period		2	1
Balance at end of period		(197)	(156)
Pension and postretirement benefit liability:
Balance at beginning of period		3	0
Change in unfunded pension and postretirement liability, net of tax of less then $1 million and less then $1 million, respectively		1	0
Total other comprehensive income for period		1	0
Balance at end of period		4	0
Total accumulated other comprehensive loss at end of period		(698)	(415)	$ (431)
AOCI included in pretax unrealized losses						$ 234
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
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

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

[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.