Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,091	$ 1,222
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,300	1,409
Allowance for credit losses on TDR Loans		33	48
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[4]	363	482
Unpaid Principal Balance of TDR Loans	[2],[3],[4]	443	548
Allowance for credit losses on TDR Loans		24	37
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	0	32
Unpaid Principal Balance of TDR Loans	[2],[3]	0	33
Allowance for credit losses on TDR Loans		0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	244	300
Unpaid Principal Balance of TDR Loans	[2],[3]	320	363
Allowance for credit losses on TDR Loans		24	37
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	119	150
Unpaid Principal Balance of TDR Loans	[2],[3]	123	152
Allowance for credit losses on TDR Loans		0	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	728	740
Unpaid Principal Balance of TDR Loans	[2],[3]	857	861
Allowance for credit losses on TDR Loans		9	11
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	693	708
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	789	797
Allowance for credit losses on TDR Loans		7	9
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	31	27
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	64	59
Allowance for credit losses on TDR Loans		1	1
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	4	5
Unpaid Principal Balance of TDR Loans	[2],[3]	4	5
Allowance for credit losses on TDR Loans		$ 1	$ 1

[1] At September 30, 2017 and December 31, 2016, the carrying value of TDR Loans includes $514 million and $645 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 $490 million and $571 million at September 30, 2017 and December 31, 2016, 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] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $223 million and $184 million at September 30, 2017 and December 31, 2016, respectively.

[5] At September 30, 2017 and December 31, 2016, the carrying value of consumer mortgage TDR Loans held for investment includes $662 million and $672 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.