Retained Earnings and Regulatory Capital Requirements - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
HSBC USA Inc [Member]
Total capital ratio:
Capital Amount	$ 20,482		$ 20,764
Well-Capitalized Minimum Ratio, Total capital	10.00%	[1]	10.00%	[1]
Actual Ratio, Total Capital	16.30%		19.52%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital	14,652		14,480
Well-Capitalized Minimum Ratio, Tier 1 capital	6.00%	[1]	6.00%	[1]
Actual Ratio, Tier 1 Capital	11.66%		13.61%
Tier 1 common ratio:
Capital Amount, Tier 1 common capital	12,544		12,373
Well-Capitalized Minimum Ratio, Tier 1 common capital	5.00%	[1],[2]	5.00%	[1],[2]
Actual Ratio, Tier 1 common capital	9.98%		11.63%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital	14,652		14,480
Well-Capitalized Minimum Ratio, Tier 1 leverage capital	3.00%	[1],[3]	3.00%	[1],[3]
Actual Ratio, Tier 1 leverage capital	8.40%		7.70%
Risk weighted assets:
Risk Weighted Assets	125,641		106,395
HSBC Bank USA, National Association [Member]
Total capital ratio:
Capital Amount	21,374		21,464
Well-Capitalized Minimum Ratio, Total capital	10.00%	[1]	10.00%	[1]
Actual Ratio, Total Capital	18.01%		21.07%
Tier 1 capital ratio:
Capital Amount, Tier 1 capital	15,817		15,482
Well-Capitalized Minimum Ratio, Tier 1 capital	6.00%	[1]	6.00%	[1]
Actual Ratio, Tier 1 Capital	13.33%		15.20%
Tier 1 common ratio:
Capital Amount, Tier 1 common capital	15,817		15,482
Well-Capitalized Minimum Ratio, Tier 1 common capital	5.00%	[1]	5.00%	[1]
Actual Ratio, Tier 1 common capital	13.33%		15.20%
Tier 1 leverage ratio:
Capital Amount, Tier 1 leverage capital	15,817		15,482
Well-Capitalized Minimum Ratio, Tier 1 leverage capital	5.00%	[1]	5.00%	[1]
Actual Ratio, Tier 1 leverage capital	9.30%		8.43%
Risk weighted assets:
Risk Weighted Assets	$ 118,656		$ 101,865

[1] HSBC USA Inc 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 minimum 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 is no Tier 1 common ratio component in the definition of a well-capitalized bank holding company. The ratio shown is the required minimum Tier 1 common ratio as included in the Federal Reserve Board's final rule regarding capital plans for U.S. bank holding companies with total consolidated assets of $50 billion or more.

[3] There is no Tier 1 leverage ratio component in the definition of a well-capitalized bank holding company. The ratio shown is the minimum required ratio.