Financial Statements of HSBC USA Inc. (Parent) Statement of Comprehensive Income(Loss) (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2018	[1]	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		$ 177	$ 269	$ (238)	$ (773)	$ 94	$ 204	$ 296	$ 320	$ (179)	$ 129
Net change in unrealized gains (losses), net of tax:
Investment securities										(220)	211	(227)
Fair value option liabilities attributable to our own credit spread										324	(193)	0
Pension and post-retirement benefit plans										8	2	3
Total other comprehensive income (loss)										152	13	(211)
Comprehensive income (loss)										472	(166)	(82)
HSBC USA Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)										320	(179)	129
Net change in unrealized gains (losses), net of tax:
Investment securities										(220)	211	(227)
Fair value option liabilities attributable to our own credit spread										324	(193)	0
Derivatives designated as cash flow hedges										40	(7)	13
Pension and post-retirement benefit plans										8	2	3
Total other comprehensive income (loss)										152	13	(211)
Comprehensive income (loss)										$ 472	$ (166)	$ (82)

[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.