Fair Value Measurements - Carrying Value and Estimated Fair Value of Financial Instruments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value Of Other Financial Instrument [Line Items]
Derivatives	$ 6,094	$ 7,569
Derivatives	6,332	7,348
Level 1 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Short-term financial assets	1,013	961
Federal funds sold and securities purchased under resale agreements	0	0
Non-derivative trading assets	1,958	1,344
Derivatives	22	26
Securities	21,744	39,553
Commercial loans, net of allowance for credit losses	0	0
Consumer loans, net of allowance for credit losses	0	0
Short-term financial liabilities	0	0
Deposits designated under fair value option	0	0
Non-derivative trading liabilities	412	308
Derivatives	5	5
Long-term debt	0	0
Long-term debt designated under fair value option	0	0
Level 1 [Member] \| Commercial Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Commercial loans designated under fair value option and held for sale	0	0
Loans held for sale	0	0
Level 1 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	0	0
Level 1 [Member] \| Consumer Loans [Member] \| Other consumer [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	0	0
Level 1 [Member] \| Deposits Without Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	0	0
Level 1 [Member] \| Deposits With Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	0	0
Level 2 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Short-term financial assets	26,379	19,614
Federal funds sold and securities purchased under resale agreements	1,096	2,119
Non-derivative trading assets	12,397	18,924
Derivatives	6,029	7,467
Securities	20,278	16,852
Commercial loans, net of allowance for credit losses	0	0
Consumer loans, net of allowance for credit losses	0	0
Short-term financial liabilities	11,882	19,154
Deposits designated under fair value option	5,632	5,406
Non-derivative trading liabilities	3,213	3,826
Derivatives	6,305	7,294
Long-term debt	16,919	15,729
Long-term debt designated under fair value option	8,064	6,686
Level 2 [Member] \| Commercial Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Commercial loans designated under fair value option and held for sale	69	58
Loans held for sale	154	18
Level 2 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	17	36
Level 2 [Member] \| Consumer Loans [Member] \| Other consumer [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	0	0
Level 2 [Member] \| Deposits Without Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	98,939	102,584
Level 2 [Member] \| Deposits With Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	2,204	2,289
Level 3 [Member]
Fair Value Of Other Financial Instrument [Line Items]
Short-term financial assets	51	51
Federal funds sold and securities purchased under resale agreements	0	0
Non-derivative trading assets	3,098	2,635
Derivatives	43	76
Securities	0	0
Commercial loans, net of allowance for credit losses	55,333	49,897
Consumer loans, net of allowance for credit losses	17,311	16,051
Short-term financial liabilities	51	51
Deposits designated under fair value option	1,905	2,334
Non-derivative trading liabilities	0	0
Derivatives	22	49
Long-term debt	0	0
Long-term debt designated under fair value option	632	900
Level 3 [Member] \| Commercial Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Commercial loans designated under fair value option and held for sale	0	0
Loans held for sale	0	0
Level 3 [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	21	56
Level 3 [Member] \| Consumer Loans [Member] \| Other consumer [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	66	63
Level 3 [Member] \| Deposits Without Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	0	0
Level 3 [Member] \| Deposits With Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	0	0
Carrying Value [Member]
Fair Value Of Other Financial Instrument [Line Items]
Short-term financial assets	27,443	20,626
Federal funds sold and securities purchased under resale agreements	1,096	2,119
Non-derivative trading assets	17,453	22,903
Derivatives	6,094	7,569
Securities	41,934	56,368
Commercial loans, net of allowance for credit losses	53,650	48,186
Consumer loans, net of allowance for credit losses	19,158	18,903
Short-term financial liabilities	11,933	19,205
Deposits designated under fair value option	7,537	7,740
Non-derivative trading liabilities	3,625	4,134
Derivatives	6,332	7,348
Long-term debt	16,338	15,261
Long-term debt designated under fair value option	8,696	7,586
Carrying Value [Member] \| Commercial Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Commercial loans designated under fair value option and held for sale	69	58
Loans held for sale	154	18
Carrying Value [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	38	91
Carrying Value [Member] \| Consumer Loans [Member] \| Other consumer [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	66	63
Carrying Value [Member] \| Deposits Without Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	98,939	102,584
Carrying Value [Member] \| Deposits With Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	2,201	2,284
Estimate of fair value [Member]
Fair Value Of Other Financial Instrument [Line Items]
Short-term financial assets	27,443	20,626
Federal funds sold and securities purchased under resale agreements	1,096	2,119
Non-derivative trading assets	17,453	22,903
Derivatives	6,094	7,569
Securities	42,022	56,405
Commercial loans, net of allowance for credit losses	55,333	49,897
Consumer loans, net of allowance for credit losses	17,311	16,051
Short-term financial liabilities	11,933	19,205
Deposits designated under fair value option	7,537	7,740
Non-derivative trading liabilities	3,625	4,134
Derivatives	6,332	7,348
Long-term debt	16,919	15,729
Long-term debt designated under fair value option	8,696	7,586
Estimate of fair value [Member] \| Commercial Loans [Member]
Fair Value Of Other Financial Instrument [Line Items]
Commercial loans designated under fair value option and held for sale	69	58
Loans held for sale	154	18
Estimate of fair value [Member] \| Consumer Loans [Member] \| Residential Mortgage [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	38	92
Estimate of fair value [Member] \| Consumer Loans [Member] \| Other consumer [Member]
Fair Value Of Other Financial Instrument [Line Items]
Loans held for sale	66	63
Estimate of fair value [Member] \| Deposits Without Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	98,939	102,584
Estimate of fair value [Member] \| Deposits With Fixed Maturities [Member]
Fair Value Of Other Financial Instrument [Line Items]
Deposits	$ 2,204	$ 2,289

Fair Value of Commercial Syndicated Loans Held For Sale

No definition available.

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- Definition

Interest bearing deposits domestic fair value option disclosure.

+ References

No definition available.

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- Definition

This element represents the portion of long term debt that the entity has elected to value at fair value under the fair value option.

+ References

No definition available.

+ Details

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Namespace Prefix:	hsbcusa_
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Period Type:	instant

- Definition

Non-derivative trading assets fair value disclosure.

+ References

No definition available.

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- Definition

Non derivative trading liability.

+ References

No definition available.

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- Definition

Securities, Fair Value Disclosure

+ References

No definition available.

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- Definition

Short-term financial assets fair value disclosure.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

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-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of federal funds sold and securities borrowed or purchased under agreements to resell.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(3)(a))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of commercial loans and leases receivable including, but not limited to, commercial and industrial loans, agricultural loans, mortgage loans, promissory notes, interbank and other loans to financial institutions, draws against credit facilities, trade financing, lease financings. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

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-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5066-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of consumer loans, including, but not limited to, mortgage, home equity, vehicles, credit card, installment, and lease financing. Excludes loans and leases covered under loss sharing agreements.

+ References

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

+ References

No definition available.

+ Details

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.

+ References

No definition available.

+ Details

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