Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 16,979	$ 17,428
Well Capitalized Ratio, Common Equity Tier 1	[1],[2]	4.50%	4.50%
Actual Ratio, Common equity Tier 1		13.60%	14.20%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 18,244	$ 18,696
Well-Capitalized Ratio, Tier 1 capital	[1]	6.00%	6.00%
Actual Ratio, Tier 1 Capital		14.60%	15.30%
Total capital ratio:
Capital Amount		$ 21,839	$ 22,565
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		17.50%	18.40%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 18,244	$ 18,696
Well-Capitalized Ratio, Tier 1 leverage capital	[1],[2]	4.00%	4.00%
Actual Ratio, Tier 1 leverage capital		9.90%	9.90%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 18,244
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%
Actual Ratio, Supplementary Leverage Ratio		7.30%
Risk weighted assets:
Risk Weighted Assets		$ 124,937	$ 122,584
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[4]	183,539	188,774
Total Leverage Exposure [Abstract]
Total leverage exposure	[5]	250,718
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 18,843	$ 19,294
Well Capitalized Ratio, Common Equity Tier 1	[1]	6.50%	6.50%
Actual Ratio, Common equity Tier 1		15.40%	16.70%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 21,343	$ 21,786
Well-Capitalized Ratio, Tier 1 capital	[1]	8.00%	8.00%
Actual Ratio, Tier 1 Capital		17.40%	18.80%
Total capital ratio:
Capital Amount		$ 24,996	$ 25,522
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		20.40%	22.10%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 21,343	$ 21,786
Well-Capitalized Ratio, Tier 1 leverage capital	[1]	5.00%	5.00%
Actual Ratio, Tier 1 leverage capital		11.80%	11.70%
Supplementary Leverage Ratio [Abstract]
Capital Amount, Supplementary Leverage Ratio		$ 21,343
Well-capitalized Ratio, Supplementary Leverage Ratio	[1],[3]	3.00%
Actual Ratio, Supplementary Leverage Ratio		8.60%
Risk weighted assets:
Risk Weighted Assets		$ 122,752	$ 115,667
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[4]	181,179	$ 186,551
Total Leverage Exposure [Abstract]
Total leverage exposure	[5]	$ 247,662

[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 minimum ratios.

[3] Beginning January 1, 2018, HSBC USA and HSBC Bank USA are required to maintain the regulatory minimum SLR of 3 percent. There is no SLR component in the definition of a well-capitalized banking institution.

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

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