Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		$ 648	$ 624	$ 680	$ 606
Provision for credit losses		47	23	94	124
Charge offs		(24)	(29)	(126)	(144)
Recoveries		7	13	30	45
Net (charge offs) recoveries		(17)	(16)	(96)	(99)
Balance at end of period		678	631	678	631
Ending balance: collectively evaluated for impairment						$ 623		$ 566
Ending balance: individually evaluated for impairment						55		65
Total allowances for credit losses		648	624	680	606	678	$ 680	631
Collectively evaluated for impairment	[1]					82,868		71,372
Individually evaluated for impairment	[2]					556		485
Loans carried at the lower of amortized cost or fair value less cost to sell						1,518		1,582
Total loans						84,942	77,741	73,439
Commercial Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans						64,752	58,087
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		89	94	89	108
Provision for credit losses		(3)	(9)	(4)	(1)
Charge offs		0	0	(2)	(24)
Recoveries		1	1	4	3
Net (charge offs) recoveries		1	1	2	(21)
Balance at end of period		87	86	87	86
Ending balance: collectively evaluated for impairment						86		84
Ending balance: individually evaluated for impairment						1		2
Total allowances for credit losses		89	94	89	108	87	89	86
Collectively evaluated for impairment	[1]					11,980		9,459
Individually evaluated for impairment	[2]					116		202
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						12,096	10,300	9,661
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		260	180	275	112
Provision for credit losses		32	36	54	111
Charge offs		(11)	(5)	(55)	(16)
Recoveries		2	2	9	6
Net (charge offs) recoveries		(9)	(3)	(46)	(10)
Balance at end of period		283	213	283	213
Ending balance: collectively evaluated for impairment						269		206
Ending balance: individually evaluated for impairment						14		7
Total allowances for credit losses		260	180	275	112	283	275	213
Collectively evaluated for impairment	[1]					19,325		17,318
Individually evaluated for impairment	[2]					189		37
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						19,514	17,819	17,355
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		130	96	107	68
Provision for credit losses		16	0	39	36
Charge offs		0	0	0	(8)
Recoveries		0	0	0	0
Net (charge offs) recoveries		0	0	0	(8)
Balance at end of period		146	96	146	96
Ending balance: collectively evaluated for impairment						146		96
Ending balance: individually evaluated for impairment						0		0
Total allowances for credit losses		130	96	107	68	146	107	96
Collectively evaluated for impairment	[1]					29,787		24,001
Individually evaluated for impairment	[2]					21		0
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						29,808	26,387	24,001
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		18	13	21	20
Provision for credit losses		(1)	1	(3)	(12)
Charge offs		0	0	(1)	0
Recoveries		0	3	0	9
Net (charge offs) recoveries		0	3	(1)	9
Balance at end of period		17	17	17	17
Ending balance: collectively evaluated for impairment						17		16
Ending balance: individually evaluated for impairment						0		1
Total allowances for credit losses		18	13	21	20	17	21	17
Collectively evaluated for impairment	[1]					3,328		3,035
Individually evaluated for impairment	[2]					6		10
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						3,334	3,581	3,045
Consumer Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans						20,190	19,654
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		83	149	107	186
Provision for credit losses		(3)	(11)	(9)	(20)
Charge offs		(3)	(9)	(27)	(45)
Recoveries		2	3	8	11
Net (charge offs) recoveries		(1)	(6)	(19)	(34)
Balance at end of period		79	132	79	132
Ending balance: collectively evaluated for impairment						41		81
Ending balance: individually evaluated for impairment						38		51
Total allowances for credit losses		83	149	107	186	79	107	132
Collectively evaluated for impairment	[1]					15,801		14,662
Individually evaluated for impairment	[2]					214		224
Loans carried at the lower of amortized cost or fair value less cost to sell						1,445		1,516
Total loans						17,460	16,661	16,402
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		27	42	32	49
Provision for credit losses		(2)	(5)	(4)	(11)
Charge offs		(1)	(3)	(6)	(11)
Recoveries		1	2	3	9
Net (charge offs) recoveries		0	(1)	(3)	(2)
Balance at end of period		25	36	25	36
Ending balance: collectively evaluated for impairment						24		34
Ending balance: individually evaluated for impairment						1		2
Total allowances for credit losses		27	42	32	49	25	32	36
Collectively evaluated for impairment	[1]					1,561		1,753
Individually evaluated for impairment	[2]					5		5
Loans carried at the lower of amortized cost or fair value less cost to sell						73		66
Total loans						1,639	1,784	1,824
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		33	39	39	50
Provision for credit losses		7	9	14	18
Charge offs		(8)	(10)	(24)	(33)
Recoveries		1	2	4	5
Net (charge offs) recoveries		(7)	(8)	(20)	(28)
Balance at end of period		33	40	33	40
Ending balance: collectively evaluated for impairment						32		38
Ending balance: individually evaluated for impairment						1		2
Total allowances for credit losses		33	39	39	50	33	39	40
Collectively evaluated for impairment	[1]					677		679
Individually evaluated for impairment	[2]					5		7
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						682	720	686
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		8	11	10	13
Provision for credit losses		1	2	7	3
Charge offs		(1)	(2)	(11)	(7)
Recoveries		0	0	2	2
Net (charge offs) recoveries		(1)	(2)	(9)	(5)
Balance at end of period		8	11	8	11
Ending balance: collectively evaluated for impairment						8		11
Ending balance: individually evaluated for impairment						0		0
Total allowances for credit losses		$ 8	$ 11	$ 10	$ 13	8	10	11
Collectively evaluated for impairment	[1]					409		465
Individually evaluated for impairment	[2]					0		0
Loans carried at the lower of amortized cost or fair value less cost to sell						0		0
Total loans						$ 409	$ 489	$ 465

[1] Global Banking includes loans to HSBC affiliates totaling $5,447 million and $4,821 million at September 30, 2015 and December 31, 2014, respectively, for which we do not carry an associated allowance for credit losses.

[2] For consumer loans and certain small business loans, these amounts represent TDR Loans for which we evaluate reserves using a discounted cash flow methodology. Each loan is individually identified as a TDR Loan and then grouped together with other TDR Loans with similar characteristics. The discounted cash flow analysis is then applied to these groups of TDR Loans. Loans individually evaluated for impairment exclude TDR loans that are carried at the lower of amortized cost or fair value of the collateral less cost to sell which totaled $858 million and $741 million at September 30, 2015 and 2014, respectively.