Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) $ in Millions		Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common equity Tier 1		$ 15,808	$ 15,876
Well-Capitalized Ratio, Common equity Tier 1	[1],[2]	0.045	0.045
Actual Ratio, Common equity Tier 1		0.142	0.131
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 17,073	$ 17,141
Well-Capitalized Ratio, Tier 1 capital	[1]	0.060	0.060
Actual Ratio, Tier 1 capital		0.153	0.141
Total capital ratio:
Capital Amount, Total capital		$ 20,641	$ 19,743
Well-Capitalized Ratio, Total capital	[1]	0.100	0.100
Actual Ratio, Total capital		0.185	0.163
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 17,073	$ 17,141
Well-Capitalized Ratio, Tier 1 leverage capital	[1],[2]	0.040	0.040
Actual Ratio, Tier 1 leverage capital		0.084	0.099
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 17,073	$ 17,141
Well-Capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	0.030	0.030
Actual Ratio, Supplementary Leverage Ratio		0.076	0.069
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual	[4]	$ 111,467	$ 121,407
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	202,252	173,270
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	225,722	247,590
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common equity Tier 1		$ 18,054	$ 18,043
Well-Capitalized Ratio, Common equity Tier 1	[1]	0.065	0.065
Actual Ratio, Common equity Tier 1		0.165	0.152
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 20,554	$ 20,543
Well-Capitalized Ratio, Tier 1 capital	[1]	0.080	0.080
Actual Ratio, Tier 1 capital		0.188	0.173
Total capital ratio:
Capital Amount, Total capital		$ 23,220	$ 22,724
Well-Capitalized Ratio, Total capital	[1]	0.100	0.100
Actual Ratio, Total capital		0.212	0.192
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 20,554	$ 20,543
Well-Capitalized Ratio, Tier 1 leverage capital	[1]	0.050	0.050
Actual Ratio, Tier 1 leverage capital		0.102	0.120
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 20,554	$ 20,543
Well-Capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	0.030	0.030
Actual Ratio, Supplementary Leverage Ratio		0.092	0.084
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual	[4]	$ 109,565	$ 118,618
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	202,404	170,722
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	$ 223,754	$ 244,008

[1] HSBC USA and HSBC Bank USA are categorized as "well-capitalized," as defined by their principal regulators. To be categorized as well-capitalized under regulatory guidelines, a banking institution must have the ratios reflected in the above table, and must not be subject to a directive, order, or written agreement to meet and maintain specific capital levels.

[2] There are no common equity Tier 1 or Tier 1 leverage ratio components in the definition of a well-capitalized bank holding company. The ratios shown are the regulatory minimums.

[3] There is no SLR component in the definition of a well-capitalized banking institution. The ratios shown are the regulatory minimums.

[4] Calculated using the generally-applicable Standardized Approach.

[5] Represents the Tier 1 leverage ratio denominator which reflects quarterly average assets adjusted for amounts permitted to be deducted from Tier 1 capital.

[6] Represents the SLR denominator which includes adjusted quarterly average assets plus certain off-balance sheet exposures.