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
		Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018		Dec. 31, 2017		Mar. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		$ 681	$ 1,017
Provision charged (credited) to income		(71)	(77)
Charge offs		(41)	(36)
Recoveries		30	17
Net (charge offs) recoveries		(11)	(19)
Balance at end of period		599	921
Ending balance: collectively evaluated for impairment				$ 490				$ 645
Ending balance: individually evaluated for impairment				109				276
Total allowances for credit losses		681	1,017	599		$ 681		921
Collectively evaluated for impairment	[1]			65,696				67,545
Individually evaluated for impairment	[2]			682				993
Loans carried at the lower of amortized cost or fair value less cost to sell				960				1,050
Total loans				67,338		72,563		69,588
Commercial Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans				47,739		53,035
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		82	92
Provision charged (credited) to income		10	1
Charge offs		0	(1)
Recoveries		0	0
Net (charge offs) recoveries		0	(1)
Balance at end of period		92	92
Ending balance: collectively evaluated for impairment				91				86
Ending balance: individually evaluated for impairment				1				6
Total allowances for credit losses		82	92	92		82		92
Collectively evaluated for impairment	[1]			11,042				10,758
Individually evaluated for impairment	[2]			12				44
Loans carried at the lower of amortized cost or fair value less cost to sell				0				0
Total loans				11,054		10,533		10,802
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		244	317
Provision charged (credited) to income		(34)	(42)
Charge offs		(25)	(21)
Recoveries		22	12
Net (charge offs) recoveries		(3)	(9)
Balance at end of period		207	266
Ending balance: collectively evaluated for impairment				168				230
Ending balance: individually evaluated for impairment				39				36
Total allowances for credit losses		244	317	207		244		266
Collectively evaluated for impairment	[1]			12,532				12,828
Individually evaluated for impairment	[2]			286				324
Loans carried at the lower of amortized cost or fair value less cost to sell				0				0
Total loans				12,818		12,504		13,152
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		264	508
Provision charged (credited) to income		(49)	(39)
Charge offs		(5)	0
Recoveries		1	1
Net (charge offs) recoveries		(4)	1
Balance at end of period		211	470
Ending balance: collectively evaluated for impairment				149				247
Ending balance: individually evaluated for impairment				62				223
Total allowances for credit losses		264	508	211		264		470
Collectively evaluated for impairment	[1]			18,967				21,974
Individually evaluated for impairment	[2]			322				553
Loans carried at the lower of amortized cost or fair value less cost to sell				0				0
Total loans				19,289	[3],[4]	20,088	[3],[4]	22,527
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		18	13
Provision charged (credited) to income		1	2
Charge offs		0	0
Recoveries		0	0
Net (charge offs) recoveries		0	0
Balance at end of period		19	15
Ending balance: collectively evaluated for impairment				19				14
Ending balance: individually evaluated for impairment				0				1
Total allowances for credit losses		18	13	19		18		15
Collectively evaluated for impairment	[1]			4,578				3,508
Individually evaluated for impairment	[2]			0				7
Loans carried at the lower of amortized cost or fair value less cost to sell				0				0
Total loans				4,578	[4]	9,910	[4]	3,515
Consumer Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans				19,599		19,528
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		25	26
Provision charged (credited) to income		(10)	0
Charge offs		0	(3)
Recoveries		3	1
Net (charge offs) recoveries		3	(2)
Balance at end of period		18	24
Ending balance: collectively evaluated for impairment				13				16
Ending balance: individually evaluated for impairment				5				8
Total allowances for credit losses		25	26	18		25		24
Collectively evaluated for impairment	[1]			16,375				16,181
Individually evaluated for impairment	[2]			55				57
Loans carried at the lower of amortized cost or fair value less cost to sell				895				981
Total loans				17,325		17,273		17,219
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		11	20
Provision charged (credited) to income		(1)	(2)
Charge offs		(2)	(2)
Recoveries		2	1
Net (charge offs) recoveries		0	(1)
Balance at end of period		10	17
Ending balance: collectively evaluated for impairment				9				16
Ending balance: individually evaluated for impairment				1				1
Total allowances for credit losses		11	20	10		11		17
Collectively evaluated for impairment	[1]			1,071				1,271
Individually evaluated for impairment	[2]			3				4
Loans carried at the lower of amortized cost or fair value less cost to sell				65				69
Total loans				1,139		1,191		1,344
Consumer Loans [Member] | Credit Cards [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		32	34
Provision charged (credited) to income		12	3
Charge offs		(8)	(8)
Recoveries		2	2
Net (charge offs) recoveries		(6)	(6)
Balance at end of period		38	31
Ending balance: collectively evaluated for impairment				37				30
Ending balance: individually evaluated for impairment				1				1
Total allowances for credit losses		32	34	38		32		31
Collectively evaluated for impairment	[1]			766				642
Individually evaluated for impairment	[2]			4				4
Loans carried at the lower of amortized cost or fair value less cost to sell				0				0
Total loans				770		721		646
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		5	7
Provision charged (credited) to income		0	0
Charge offs		(1)	(1)
Recoveries		0	0
Net (charge offs) recoveries		(1)	(1)
Balance at end of period		4	6
Ending balance: collectively evaluated for impairment				4				6
Ending balance: individually evaluated for impairment				0				0
Total allowances for credit losses		$ 5	$ 7	4		5		6
Collectively evaluated for impairment	[1]			365				383
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				$ 365		$ 343		$ 383

[1] Other commercial includes loans to HSBC affiliates totaling $1,920 million and $939 million at March 31, 2018 and 2017, 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 $644 million and $681 million at March 31, 2018 and 2017, respectively.

[3] Represents large multinational firms including globally focused U.S. corporate and financial institutions, U.S. dollar lending to multinational banking clients managed by HSBC on a global basis and complex large business clients supported by Global Banking and Markets relationship managers.

[4] ncludes loans to HSBC affiliates which totaled $1,920 million and $6,750 million at March 31, 2018 and December 31, 2017, respectively. See Note 14, "Related Party Transactions," for additional information regarding loans to HSBC affiliates.