Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) $ in Millions		Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common equity Tier 1	[1]	$ 16,154	$ 15,891
Well-Capitalized Ratio, Common equity Tier 1	[1],[2],[3]	0.045	0.045
Actual Ratio, Common equity Tier 1	[1]	0.151	0.145
Tier 1 capital ratio:
Capital Amount, Tier 1 capital	[1]	$ 17,419	$ 17,156
Well-Capitalized Ratio, Tier 1 capital	[1],[2]	0.060	0.060
Actual Ratio, Tier 1 capital	[1]	0.162	0.156
Total capital ratio:
Capital Amount, Total capital	[1]	$ 20,080	$ 20,680
Well-Capitalized Ratio, Total capital	[1],[2]	0.100	0.100
Actual Ratio, Total capital	[1]	0.187	0.188
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 17,419	$ 17,156
Well-Capitalized Ratio, Tier 1 leverage capital	[2],[3]	0.040	0.040
Actual Ratio, Tier 1 leverage capital		0.088	0.086
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 17,419	$ 17,156
Well-Capitalized Ratio, Supplementary Leverage Ratio	[2],[4]	0.030	0.030
Actual Ratio, Supplementary Leverage Ratio		0.083	0.078
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual	[1],[5]	$ 107,232	$ 109,809
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[6]	197,127	198,698
Total Leverage Exposure [Abstract]
Total leverage exposure	[7]	209,808	$ 221,216
HSBC USA Inc [Member] | As Previously Reported [Member]
Common equity Tier 1 ratio:
Actual Ratio, Common equity Tier 1			0.147
Tier 1 capital ratio:
Actual Ratio, Tier 1 capital			0.159
Total capital ratio:
Actual Ratio, Total capital			0.192
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual			$ 107,808
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common equity Tier 1	[1]	$ 18,374	$ 18,180
Well-Capitalized Ratio, Common equity Tier 1	[1],[2]	0.065	0.065
Actual Ratio, Common equity Tier 1	[1]	0.172	0.164
Tier 1 capital ratio:
Capital Amount, Tier 1 capital	[1]	$ 20,874	$ 20,680
Well-Capitalized Ratio, Tier 1 capital	[1],[2]	0.080	0.080
Actual Ratio, Tier 1 capital	[1]	0.196	0.187
Total capital ratio:
Capital Amount, Total capital	[1]	$ 23,314	$ 23,303
Well-Capitalized Ratio, Total capital	[1],[2]	0.100	0.100
Actual Ratio, Total capital	[1]	0.218	0.211
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 20,874	$ 20,680
Well-Capitalized Ratio, Tier 1 leverage capital	[2]	0.050	0.050
Actual Ratio, Tier 1 leverage capital		0.105	0.103
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 20,874	$ 20,680
Well-Capitalized Ratio, Supplementary Leverage Ratio	[2],[4]	0.030	0.030
Actual Ratio, Supplementary Leverage Ratio		0.099	0.093
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual	[1],[5]	$ 106,758	$ 110,682
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[6]	198,580	200,026
Total Leverage Exposure [Abstract]
Total leverage exposure	[7]	$ 210,105	$ 221,334
HSBC Bank USA [Member] | As Previously Reported [Member]
Common equity Tier 1 ratio:
Actual Ratio, Common equity Tier 1			0.172
Tier 1 capital ratio:
Actual Ratio, Tier 1 capital			0.196
Total capital ratio:
Actual Ratio, Total capital			0.220
Risk weighted assets:
Banking Regulation, Risk-Weighted Assets, Actual			$ 105,681

[1] During the first quarter of 2021, it was determined that certain collateral did not qualify for risk-weighted asset reduction purposes under U.S. capital rules. As a result, reported risk-weighted assets were understated and reported Common equity Tier 1 capital, Tier 1 capital and Total capital ratios were overstated at HSBC USA and HSBC Bank USA at December 31, 2020. We have revised December 31, 2020 amounts to conform to the current period presentation.

[2] 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 maintain capital equal to or in excess of 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.

[3] 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.

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

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

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

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