Financial Statements of HSBC USA Inc. (Parent) Statement of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended										12 Months Ended
		Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net income (loss)		$ 112	[1]	$ 177	$ 269	$ (238)	$ (773)		$ 94	$ 204	$ 296	$ 320	$ (179)		$ 129
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization												44	51		60
Net change in other assets and liabilities												(1,830)	(462)		1,723
Net cash provided by (used in) operating activities												(8,185)	105		(6)
Cash flows from investing activities
Other, net												50	549		114
Net cash provided by (used in) investing activities												23,375	5,318		(1,339)
Cash flows from financing activities
Net change in short-term borrowings												(469)	(449)		110
Issuance of long-term debt, net of issuance costs												11,923	5,158		8,694
Repayment of long-term debt												(14,704)	(8,948)		(4,869)
Preferred stock issuance												0	0		1,265
Preferred stock redemption												0	0		(1,265)
Other increases (decreases) in capital surplus												6	(18)		(21)
Dividends paid												(76)	(77)		(67)
Net cash provided by (used in) financing activities												(10,248)	(14,624)		14,372
Net change in cash and due from banks and interest bearing deposits with banks(1)												4,942	(9,201)		13,027
Cash and due from banks and interest bearing deposits with banks at beginning of period	[2]					12,272					21,473	12,272	21,473		8,446
Cash and and due from banks and interest bearing deposits with banks at end of period	[2]	17,214					12,272					17,214	12,272		21,473
Cash paid for:
Interest												2,350	1,816		1,446
HSBC USA Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)												320	(179)		129
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization												8	12		14
Net change in other assets and liabilities												(1,277)	481		271
Undistributed loss (gain) of subsidiaries												(380)	369		(7)
Other, net												16	5		(4)
Net cash provided by (used in) operating activities												(1,313)	688		403
Cash flows from investing activities
Sales and maturities of securities												0	1		254
Net change in investments in and receivables due from subsidiaries												(1,045)	2,843		(496)
Net change in receivables from other HSBC affiliates												4,211	(2,882)		887
Other, net												10	403		(24)
Net cash provided by (used in) investing activities												3,176	365		621
Cash flows from financing activities
Net change in payables to other HSBC affiliates												(19)	(2)		19
Net change in short-term borrowings												1,378	(97)		(727)
Issuance of long-term debt, net of issuance costs												5,571	4,826		3,587
Repayment of long-term debt												(8,722)	(5,684)		(3,815)
Preferred stock issuance												0	0		1,265
Preferred stock redemption												0	0		(1,265)
Other increases (decreases) in capital surplus												6	(18)		(21)
Dividends paid												(76)	(77)		(67)
Net cash provided by (used in) financing activities												(1,862)	(1,052)		(1,024)
Net change in cash and due from banks and interest bearing deposits with banks(1)	[3]											1	1		0
Cash and due from banks and interest bearing deposits with banks at beginning of period	[3]					$ 1					$ 0	1	0		0
Cash and and due from banks and interest bearing deposits with banks at end of period		$ 2					$ 1	[3]				2	1	[3]	0	[3]
Cash paid for:
Interest												$ 649	$ 584		$ 517

[1] Fourth quarter 2018 reflects out of period adjustments which reduced net income by $56 million, $33 million of which relates to prior year periods. We concluded these adjustments were not material to the current and prior year periods.

[2] As a result of adopting new accounting guidance in 2018, we changed our presentation of cash and cash equivalents to include cash and due from banks as well as interest bearing deposits with banks, which includes restricted cash. See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," for further discussion.

[3] As a result of adopting new accounting guidance in 2018, we changed our presentation of cash and cash equivalents to include cash and due from banks as well as interest bearing deposits with banks. See Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," for further discussion.