Financial Statements of HSBC USA Inc. (Parent) Statement of Comprehensive Income(Loss) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ (773)	$ 94	$ 204	$ 296	$ (37)	$ 33	$ (21)	$ 154	$ (179)	$ 129	$ 330
Net change in unrealized gains (losses), net of tax:
Investment securities									211	(227)	(392)
Fair value option liabilities attributable to our own credit spread									(193)	0	0
Pension and post-retirement benefit plans									2	3	0
Total other comprehensive income (loss)									13	(211)	(406)
Comprehensive loss									(166)	(82)	(76)
HSBC USA Inc [Member]
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(179)	129	330
Net change in unrealized gains (losses), net of tax:
Investment securities									211	(227)	(392)
Fair value option liabilities attributable to our own credit spread									(193)	0	0
Derivatives designated as cash flow hedges									(7)	13	(14)
Pension and post-retirement benefit plans									2	3	0
Total other comprehensive income (loss)									13	(211)	(406)
Comprehensive loss									$ (166)	$ (82)	$ (76)