Financial Statements of HSBC USA Inc. (Parent) Statement of Comprehensive Income(Loss) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 148	$ 188	$ 7	$ (1,283)	$ 133	$ (250)	$ 121	$ 109	$ (940)	$ 113	$ 320
Net change in unrealized gains (losses), net of tax:
Investment securities									864	403	(220)
Fair value option liabilities attributable to our own credit spread									24	(310)	324
Derivatives designated as cash flow hedges									72	8	40
Pension and post-retirement benefit plans									(4)	(14)	8
Total other comprehensive income									956	87	152
Comprehensive income									16	200	472
HSBC USA Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(940)	113	320
Net change in unrealized gains (losses), net of tax:
Investment securities									864	403	(220)
Fair value option liabilities attributable to our own credit spread									24	(310)	324
Derivatives designated as cash flow hedges									72	8	40
Pension and post-retirement benefit plans									(4)	(14)	8
Total other comprehensive income									956	87	152
Comprehensive income									$ 16	$ 200	$ 472