Financial Statements of HSBC USA Inc. (Parent) Statement of Cash Flows (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
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization									34	43	44
Net change in other assets and liabilities									(839)	1,846	(1,830)
Net cash provided by (used in) operating activities									3,282	(4,259)	(8,185)
Cash flows from investing activities
Other, net									(248)	119	50
Net cash provided by (used in) investing activities									(10,805)	(8,429)	23,375
Cash flows from financing activities
Net change in short-term borrowings									1,293	(535)	(469)
Issuance of long-term debt, net of issuance costs									8,487	4,686	11,923
Repayment of long-term debt									(10,427)	(10,705)	(14,704)
Return of capital to parent									0	(2,400)	0
Other increases (decreases) in capital surplus									10	0	6
Dividends paid									(77)	(77)	(76)
Net cash provided by (used in) financing activities									19,396	(744)	(10,248)
Net change in cash and due from banks and interest bearing deposits with banks									11,873	(13,432)	4,942
Cash and due from banks and interest bearing deposits with banks at beginning of period				3,782				17,214	3,782	17,214	12,272
Cash and due from banks and interest bearing deposits with banks at end of period	15,655				3,782				15,655	3,782	17,214
Cash paid for:
Interest									1,518	2,921	2,350
HSBC USA Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(940)	113	320
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization									2	5	8
Net change in other assets and liabilities									256	1,463	(1,277)
Undistributed loss (gain) of subsidiaries									897	(205)	(380)
Other, net									(2)	4	16
Net cash provided by (used in) operating activities									213	1,380	(1,313)
Cash flows from investing activities
Net change in investments in and receivables due from subsidiaries									(552)	3,929	(1,045)
Net change in receivables from other HSBC affiliates									1,056	183	4,211
Other, net									0	1	10
Net cash provided by (used in) investing activities									504	4,113	3,176
Cash flows from financing activities
Net change in payables to other HSBC affiliates									(1)	4	(19)
Net change in short-term borrowings									3,522	(56)	1,378
Issuance of long-term debt, net of issuance costs									3,394	2,931	5,571
Repayment of long-term debt									(7,565)	(5,897)	(8,722)
Return of capital to parent									0	(2,400)	0
Other increases (decreases) in capital surplus									10	0	6
Dividends paid									(77)	(77)	(76)
Net cash provided by (used in) financing activities									(717)	(5,495)	(1,862)
Net change in cash and due from banks and interest bearing deposits with banks									0	(2)	1
Cash and due from banks and interest bearing deposits with banks at beginning of period				$ 0				$ 2	0	2	1
Cash and due from banks and interest bearing deposits with banks at end of period	$ 0				$ 0				0	0	2
Cash paid for:
Interest									$ 379	$ 662	$ 649