Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,352	$ 1,208
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,577	1,441
Allowance for credit losses on TDR Loans	[4]	43	56
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	270	210
Unpaid Principal Balance of TDR Loans	[2],[3]	297	252
Allowance for credit losses on TDR Loans	[4]	3	9
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	104	186
Unpaid Principal Balance of TDR Loans	[2],[3]	115	201
Allowance for credit losses on TDR Loans	[4]	0	4
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	145	24
Unpaid Principal Balance of TDR Loans	[2],[3]	160	51
Allowance for credit losses on TDR Loans	[4]	3	5
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	21	0
Unpaid Principal Balance of TDR Loans	[2],[3]	22	0
Allowance for credit losses on TDR Loans	[4]	0	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	1,082	998
Unpaid Principal Balance of TDR Loans	[2],[3]	1,280	1,189
Allowance for credit losses on TDR Loans	[4]	40	47
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	1,055	972
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	1,226	1,139
Allowance for credit losses on TDR Loans	[4]	38	43
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	22	20
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	49	44
Allowance for credit losses on TDR Loans	[4]	1	2
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	5	6
Unpaid Principal Balance of TDR Loans	[2],[3]	5	6
Allowance for credit losses on TDR Loans	[4]	$ 1	$ 2

[1] At September 30, 2015 and December 31, 2014, the carrying value reflected above includes $520 million and $485 million, respectively, of loans which are classified as nonaccrual.

[2] TDR Loans are considered to be impaired loans. For consumer loans, all such loans are considered impaired loans regardless of accrual status. For commercial loans, impaired loans include other loans in addition to TDR Loans which totaled $62 million and $85 million at September 30, 2015 and December 31, 2014, respectively.

[3] The carrying value of TDR Loans includes basis adjustments on the loans, such as unearned income, unamortized deferred fees and costs on originated loans, partial charge-offs and premiums or discounts on purchased loans.

[4] Included in the allowance for credit losses.

[5] At September 30, 2015 and December 31, 2014, the carrying value reflected above includes $858 million and $763 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.