Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses and Related Loan Balance by Product (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	$ 680		$ 606
Provision for credit losses	53		16
Charge offs	(38)		(67)
Recoveries	8		19
Net (charge offs) recoveries	(30)		(48)
Balance at end of period	703		574
Ending balance: collectively evaluated for impairment	631		497
Ending balance: individually evaluated for impairment	72		77
Total allowances for credit losses	703		574
Collectively evaluated for impairment	79,351		66,754
Individually evaluated for impairment	543	[1]	527	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	1,564		1,616
Total loans	81,458		68,897		77,741
Commercial Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans	61,647				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		108
Provision for credit losses	2		2
Charge offs	(1)		(19)
Recoveries	0		2
Net (charge offs) recoveries	(1)		(17)
Balance at end of period	90		93
Ending balance: collectively evaluated for impairment	87		84
Ending balance: individually evaluated for impairment	3		9
Total allowances for credit losses	90		93
Collectively evaluated for impairment	10,702		8,905
Individually evaluated for impairment	199	[1]	260	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	10,901		9,165		10,300
Commercial Loans [Member] | Business and Corporate Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	275		112
Provision for credit losses	24		19
Charge offs	(6)		(5)
Recoveries	2		3
Net (charge offs) recoveries	(4)		(2)
Balance at end of period	295		129
Ending balance: collectively evaluated for impairment	272		126
Ending balance: individually evaluated for impairment	23		3
Total allowances for credit losses	295		129
Collectively evaluated for impairment	19,155		15,681
Individually evaluated for impairment	96	[1]	17	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	19,251		15,698		17,819
Commercial Loans [Member] | Global Banking [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	107		68
Provision for credit losses	21		7
Charge offs	0		(8)
Recoveries	0		0
Net (charge offs) recoveries	0		(8)
Balance at end of period	128		67
Ending balance: collectively evaluated for impairment	128		67
Ending balance: individually evaluated for impairment	0		0
Total allowances for credit losses	128		67
Collectively evaluated for impairment	28,016		21,979
Individually evaluated for impairment	13	[1]	0	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	28,029		21,979		26,387
Commercial Loans [Member] | Other Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	21		20
Provision for credit losses	1		(4)
Charge offs	(1)		0
Recoveries	0		3
Net (charge offs) recoveries	(1)		3
Balance at end of period	21		19
Ending balance: collectively evaluated for impairment	20		18
Ending balance: individually evaluated for impairment	1		1
Total allowances for credit losses	21		19
Collectively evaluated for impairment	3,459		3,031
Individually evaluated for impairment	7	[1]	20	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	3,466		3,051		3,581
Consumer Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Total loans	19,811				19,654
Consumer Loans [Member] | Residential Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	107		186
Provision for credit losses	0		(16)
Charge offs	(14)		(17)
Recoveries	2		3
Net (charge offs) recoveries	(12)		(14)
Balance at end of period	95		156
Ending balance: collectively evaluated for impairment	53		96
Ending balance: individually evaluated for impairment	42		60
Total allowances for credit losses	95		156
Collectively evaluated for impairment	15,281		14,150
Individually evaluated for impairment	217	[1]	202	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	1,493		1,548
Total loans	16,991		15,900		16,661
Consumer Loans [Member] | Home Equity Mortgages [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	32		49
Provision for credit losses	(2)		2
Charge offs	(2)		(5)
Recoveries	1		6
Net (charge offs) recoveries	(1)		1
Balance at end of period	29		52
Ending balance: collectively evaluated for impairment	28		50
Ending balance: individually evaluated for impairment	1		2
Total allowances for credit losses	29		52
Collectively evaluated for impairment	1,640		1,851
Individually evaluated for impairment	5	[1]	19	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	71		68
Total loans	1,716		1,938		1,784
Consumer Loans [Member] | Credit Card [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	39		50
Provision for credit losses	3		5
Charge offs	(8)		(11)
Recoveries	1		1
Net (charge offs) recoveries	(7)		(10)
Balance at end of period	35		45
Ending balance: collectively evaluated for impairment	33		43
Ending balance: individually evaluated for impairment	2		2
Total allowances for credit losses	35		45
Collectively evaluated for impairment	674		674
Individually evaluated for impairment	6	[1]	9	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	680		683		720
Consumer Loans [Member] | Other consumer [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	10		13
Provision for credit losses	4		1
Charge offs	(6)		(2)
Recoveries	2		1
Net (charge offs) recoveries	(4)		(1)
Balance at end of period	10		13
Ending balance: collectively evaluated for impairment	10		13
Ending balance: individually evaluated for impairment	0		0
Total allowances for credit losses	10		13
Collectively evaluated for impairment	424		483
Individually evaluated for impairment	0	[1]	0	[1]
Loans carried at the lower of amortized cost or fair value less cost to sell	0		0
Total loans	$ 424		$ 483		$ 489

[1] 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 $778 million and $718 million at March 31, 2015 and 2014, respectively.