Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 15,876	$ 17,459
Well Capitalized Ratio, Common Equity Tier 1	[1],[2]	4.50%	4.50%
Actual Ratio, Common equity Tier 1		13.10%	13.60%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 17,141	$ 18,724
Well-Capitalized Ratio, Tier 1 capital	[1]	6.00%	6.00%
Actual Ratio, Tier 1 Capital		14.10%	14.60%
Total capital ratio:
Capital Amount		$ 19,743	$ 21,972
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		16.30%	17.20%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 17,141	$ 18,724
Well-Capitalized Ratio, Tier 1 leverage capital	[1],[2]	4.00%	4.00%
Actual Ratio, Tier 1 leverage capital		9.90%	11.00%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 17,141	$ 18,724
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%	3.00%
Actual Ratio, Supplementary Leverage Ratio		6.90%	7.60%
Risk weighted assets:
Risk Weighted Assets	[4]	$ 121,407	$ 127,917
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	173,270	170,565
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	247,590	245,796
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 18,043	$ 19,456
Well Capitalized Ratio, Common Equity Tier 1	[1]	6.50%	6.50%
Actual Ratio, Common equity Tier 1		15.20%	15.70%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 20,543	$ 21,956
Well-Capitalized Ratio, Tier 1 capital	[1]	8.00%	8.00%
Actual Ratio, Tier 1 Capital		17.30%	17.70%
Total capital ratio:
Capital Amount		$ 22,724	$ 25,293
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		19.20%	20.40%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 20,543	$ 21,956
Well-Capitalized Ratio, Tier 1 leverage capital	[1]	5.00%	5.00%
Actual Ratio, Tier 1 leverage capital		12.00%	13.10%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 20,543	$ 21,956
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%	3.00%
Actual Ratio, Supplementary Leverage Ratio		8.40%	9.10%
Risk weighted assets:
Risk Weighted Assets	[4]	$ 118,618	$ 124,112
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	170,722	168,154
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	$ 244,008	$ 242,264

[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 minimum ratios.

[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.