Accumulated Other Comprehensive Income (Loss) - (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		$ 16,991	$ 19,241
Total other comprehensive income (loss) for period		16	200		$ 472
Balance at end of period, common equity		17,026	16,991		19,241
Cumulative Effect, Period of Adoption, Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[1]				91
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(277)	(366)		(518)
Balance at end of period, common equity		679	(277)		(366)
Accumulated Other Comprehensive Loss [Member] | As Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(279)	(366)		(431)
Balance at end of period, common equity			(279)		(366)
Unrealized Gains (Losses) on Investment Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(114)	(519)		(299)
Net unrealized gains (losses) arising during the period, net of tax		944	459		(215)
Reclassification adjustment, net of tax	[2]	(104)	(67)		(18)
Reversal of provision for credit losses realized in net income (loss), net of tax	[3]	(2)	0		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	[4]	26	11		13
Balance at end of period, common equity		750	(114)		(519)
Unrealized Gains (Losses) on Investment Securities [Member] | As Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(116)	(519)		(250)
Balance at end of period, common equity			(116)		(519)
Unrealized Gains (Losses) on Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[5]	2
Balance at end of period, common equity	[5]		2
Tax on cumulative effect adjustment			1
Unrealized Gains (Losses) on Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[6]				4
Tax on cumulative effect adjustment						$ 2
Unrealized Gains (Losses) on Investment Securities [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[7]				(53)
Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Our Own Credit Spread [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(9)	301		(23)
Net unrealized gains (losses) arising during the period, net of tax		24	(217)		324
Reclassification adjustment, net of tax		0	(93)	[8]	0
Balance at end of period, common equity		15	(9)		301
Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Our Own Credit Spread [Member] | As Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(9)	301		(19)
Balance at end of period, common equity			(9)		301
Unrealized Gains (Losses) on Fair Value Option Liabilities Attributable to Our Own Credit Spread [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[7]				(4)
Unrealized Gains (Losses) on Derivatives Designated as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(151)	(159)		(199)
Net unrealized gains (losses) arising during the period, net of tax		64	(19)		22
Reclassification adjustment, net of tax	[9]	8	27		18
Balance at end of period, common equity		(79)	(151)		(159)
Unrealized Gains (Losses) on Derivatives Designated as Cash Flow Hedges [Member] | As Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(151)	(159)		(164)
Balance at end of period, common equity			(151)		(159)
Unrealized Gains (Losses) on Derivatives Designated as Cash Flow Hedges [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[7]				(35)
Pension and Postretirement Benefit Liability [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		(3)	11		3
Net unrealized gains (losses) arising during the period, net of tax		(4)	(14)		8
Balance at end of period, common equity		(7)	(3)		11
Pension and Postretirement Benefit Liability [Member] | As Previously Reported [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity		$ (3)	11		2
Balance at end of period, common equity			$ (3)		11
Pension and Postretirement Benefit Liability [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of period, common equity	[7]				$ 1

[1] Reflects the adoption of new accounting guidance in 2018 which resulted in a cumulative effect adjustment as of January 1, 2018 to reclassify the stranded tax effects resulting from the change in the Federal corporate income tax rate from accumulated other comprehensive income (loss) to retained earnings.

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

[3] 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).

[4] 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.

[5] See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," for additional discussion.

[6] As a result of adopting new accounting guidance in 2018, beginning January 1, 2018, changes in fair value for equity securities are recognized in net income (loss). Prior to January 1, 2018, changes in fair value for equity securities classified as available-for-sale were recognized in other comprehensive income.

[7] See Note 17, "Income Taxes," for additional discussion.

[8] In 2019, HSBC Bank USA repurchased long-term debt that was designated under fair value option. As a result, the cumulative gain attributable to our own credit spread associated with the debt was reclassified to other income in our consolidated statement of income (loss)

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