Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 15,622	$ 15,876
Well Capitalized Ratio, Common Equity Tier 1	[1],[2]	4.50%	4.50%
Actual Ratio, Common equity Tier 1		11.90%	13.10%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 16,887	$ 17,141
Well-Capitalized Ratio, Tier 1 capital	[1]	6.00%	6.00%
Actual Ratio, Tier 1 Capital		12.90%	14.10%
Total capital ratio:
Capital Amount		$ 20,932	$ 19,743
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		16.00%	16.30%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 16,887	$ 17,141
Well-Capitalized Ratio, Tier 1 leverage capital	[1],[2]	4.00%	4.00%
Actual Ratio, Tier 1 leverage capital		9.00%	9.90%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 16,887	$ 17,141
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%	3.00%
Actual Ratio, Supplementary Leverage Ratio		6.50%	6.90%
Risk weighted assets:
Risk Weighted Assets	[4]	$ 130,901	$ 121,407
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	187,962	173,270
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	258,783	247,590
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 17,814	$ 18,043
Well Capitalized Ratio, Common Equity Tier 1	[1]	6.50%	6.50%
Actual Ratio, Common equity Tier 1		14.00%	15.20%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 20,314	$ 20,543
Well-Capitalized Ratio, Tier 1 capital	[1]	8.00%	8.00%
Actual Ratio, Tier 1 Capital		15.90%	17.30%
Total capital ratio:
Capital Amount		$ 23,063	$ 22,724
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		18.10%	19.20%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 20,314	$ 20,543
Well-Capitalized Ratio, Tier 1 leverage capital	[1]	5.00%	5.00%
Actual Ratio, Tier 1 leverage capital		11.00%	12.00%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 20,314	$ 20,543
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%	3.00%
Actual Ratio, Supplementary Leverage Ratio		8.00%	8.40%
Risk weighted assets:
Risk Weighted Assets	[4]	$ 127,499	$ 118,618
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[5]	184,851	170,722
Total Leverage Exposure [Abstract]
Total leverage exposure	[6]	$ 254,907	$ 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 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.