Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,416	$ 1,453
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,596	1,678
Allowance for credit losses on TDR Loans	[4]	91	59
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	444	365
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	473	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]	91	94
Unpaid Principal Balance of TDR Loans	[2],[3]	104	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]	294	227
Unpaid Principal Balance of TDR Loans	[2],[3]	310	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]	59	44
Unpaid Principal Balance of TDR Loans	[2],[3]	59	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]	972	1,088
Unpaid Principal Balance of TDR Loans	[2],[3]	1,123	1,288
Allowance for credit losses on TDR Loans	[4]	32	35
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6],[7]	942	1,060
Unpaid Principal Balance of TDR Loans	[2],[3],[6],[7]	1,064	1,233
Allowance for credit losses on TDR Loans	[4]	30	33
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	25	23
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	54	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 March 31, 2016 and December 31, 2015, the carrying value of TDR Loans includes $622 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 $269 million and $88 million at March 31, 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 $146 million and $112 million at March 31, 2016 and December 31, 2015, respectively.

[6] At March 31, 2016 and December 31, 2015, the carrying value of consumer mortgage TDR Loans held for investment includes $771 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 March 31, 2016 reflects the impact of the transfer of certain residential mortgage TDR Loans with a carrying value of $130 million to held for sale during the first quarter of 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.