Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,254	$ 1,453
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,405	1,678
Allowance for credit losses on TDR Loans	[4]	71	59
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	521	365
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	557	390
Allowance for credit losses on TDR Loans	[4]	59	24
Commercial Loans [Member] | Construction and Other Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	45	94
Unpaid Principal Balance of TDR Loans	[2],[3]	47	106
Allowance for credit losses on TDR Loans	[4]	0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	404	227
Unpaid Principal Balance of TDR Loans	[2],[3]	437	240
Allowance for credit losses on TDR Loans	[4]	59	24
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	72	44
Unpaid Principal Balance of TDR Loans	[2],[3]	73	44
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]	733	1,088
Unpaid Principal Balance of TDR Loans	[2],[3]	848	1,288
Allowance for credit losses on TDR Loans	[4]	12	35
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6],[7]	701	1,060
Unpaid Principal Balance of TDR Loans	[2],[3],[6],[7]	785	1,233
Allowance for credit losses on TDR Loans	[4]	10	33
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	27	23
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	58	50
Allowance for credit losses on TDR Loans	[4]	1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	5	5
Unpaid Principal Balance of TDR Loans	[2],[3]	5	5
Allowance for credit losses on TDR Loans	[4]	$ 1	$ 1

[1] At September 30, 2016 and December 31, 2015, the carrying value of TDR Loans includes $610 million and $676 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 $614 million and $88 million at September 30, 2016 and December 31, 2015, 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] Additional commitments to lend to commercial borrowers whose loans have been modified in TDRs totaled $205 million and $112 million at September 30, 2016 and December 31, 2015, respectively.

[6] At September 30, 2016 and December 31, 2015, the carrying value of consumer mortgage TDR Loans held for investment includes $662 million and $881 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.

[7] The decrease in TDR Loans at September 30, 2016 reflects the impact of transfers of certain residential mortgage loans to held for sale during the nine months ended September 30, 2016. There is no allowance for credit losses associated with loans classified as held for sale as they are carried at the lower of amortized cost or fair value less cost to sell.