Regulatory Capital - Capital Amounts and Ratios in Accordance With Current Banking Regulations (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
HSBC USA Inc [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital Amount	$ 20,262	$ 21,908
Capital Amount, Tier 1 capital	13,981	15,179
Capital Amount, Tier 1 common capital	11,574	12,773
Capital Amount, Tier 1 leverage capital	13,981	15,179
Risk Weighted Assets	105,534	119,099
Well-Capitalized Minimum Ratio, Total capital	10.00%	10.00%
Well-Capitalized Minimum Ratio, Tier 1 capital	6.00%	6.00%
Well-Capitalized Minimum Ratio, Tier 1 common capital	5.00%	5.00%
Well-Capitalized Minimum Ratio, Tier 1 leverage capital	3.00%	3.00%
Actual Ratio, Total Capital	19.20%	18.39%
Actual Ratio, Tier 1 Capital	13.25%	12.74%
Actual Ratio, Tier 1 common capital	10.97%	10.72%
Actual Ratio, Tier 1 leverage capital	7.35%	7.43%
HSBC Bank USA [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital Amount	20,876	22,390
Capital Amount, Tier 1 capital	14,897	15,996
Capital Amount, Tier 1 common capital	14,897	15,996
Capital Amount, Tier 1 leverage capital	14,897	15,996
Risk Weighted Assets	$ 102,938	$ 118,688
Well-Capitalized Minimum Ratio, Total capital	10.00%	10.00%
Well-Capitalized Minimum Ratio, Tier 1 capital	6.00%	6.00%
Well-Capitalized Minimum Ratio, Tier 1 common capital	5.00%	5.00%
Well-Capitalized Minimum Ratio, Tier 1 leverage capital	5.00%	5.00%
Actual Ratio, Total Capital	20.28%	18.86%
Actual Ratio, Tier 1 Capital	14.47%	13.48%
Actual Ratio, Tier 1 common capital	14.47%	13.48%
Actual Ratio, Tier 1 leverage capital	8.01%	7.98%

Actual ratio under total capital.

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- Definition

Actual ratio, tier one capital.

+ References

No definition available.

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- Definition

Actual ratio under tier one common capital.

+ References

No definition available.

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- Definition

Actual ratio under tier one leverage capital.

+ References

No definition available.

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- Definition

Total capital required under tier one common capital.

+ References

No definition available.

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- Definition

Minimum ratio required to be well capitalized.

+ References

No definition available.

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- Definition

Well capitalized minimum ratio under tier one common capital.

+ References

No definition available.

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- Definition

The amount of total capital as defined in the regulations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 54

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998

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-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 15

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Section 50

-Paragraph 1H

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- Definition

Amount of risk-weighted assets as calculated in accordance with U.S. Federal regulatory capital standards.

+ References

No definition available.

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- Definition

Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1H

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 15

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 505

-Section 50

-Paragraph 1

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826

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- Definition

The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

+ References

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- Definition

Tier 1 Risk Based Capital as defined in the regulations.

+ References

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- Definition

The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 15

-Subparagraph 3a

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-DEP

-Paragraph 22

-IssueDate 2006-05-01

-Chapter 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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