Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
HSBC USA Inc [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 17,428	$ 17,544
Well Capitalized Ratio, Common Equity Tier 1	[1],[2]	4.50%	4.50%
Actual Ratio, Common equity Tier 1		14.20%	13.70%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 18,696	$ 18,640
Well-Capitalized Ratio, Tier 1 capital	[1]	6.00%	6.00%
Actual Ratio, Tier 1 Capital		15.30%	14.50%
Total capital ratio:
Capital Amount		$ 22,565	$ 23,549
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		18.40%	18.30%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 18,696	$ 18,640
Well-Capitalized Ratio, Tier 1 leverage capital	[1],[2]	4.00%	4.00%
Actual Ratio, Tier 1 leverage capital		9.90%	9.20%
Risk weighted assets:
Risk Weighted Assets		$ 122,584	$ 128,482
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[3]	188,774	203,000
HSBC Bank USA [Member]
Common equity Tier 1 ratio:
Capital Amount, Common Equity Tier 1		$ 19,294	$ 19,577
Well Capitalized Ratio, Common Equity Tier 1	[1]	6.50%	6.50%
Actual Ratio, Common equity Tier 1		16.70%	15.70%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital		$ 21,786	$ 21,971
Well-Capitalized Ratio, Tier 1 capital	[1]	8.00%	8.00%
Actual Ratio, Tier 1 Capital		18.80%	17.60%
Total capital ratio:
Capital Amount		$ 25,522	$ 26,325
Well-Capitalized Ratio	[1]	10.00%	10.00%
Actual Ratio, Total Capital		22.10%	21.10%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital		$ 21,786	$ 21,971
Well-Capitalized Ratio, Tier 1 leverage capital	[1]	5.00%	5.00%
Actual Ratio, Tier 1 leverage capital		11.70%	11.10%
Risk weighted assets:
Risk Weighted Assets		$ 115,667	$ 124,666
Adjusted Quarterly Average Assets [Abstract]
Adjusted quarterly average assets	[3]	$ 186,551	$ 197,944

[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] Represents the Tier 1 leverage ratio denominator which reflects quarterly average assets adjusted for amounts permitted to be deducted from Tier 1 capital for the three months ended December 31, 2017 and 2016, respectively.