Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 958	$ 1,089
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,138	1,295
Allowance for credit losses on TDR Loans	[4]	23	40
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	252	369
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	306	446
Allowance for credit losses on TDR Loans	[4]	16	31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	156	194
Unpaid Principal Balance of TDR Loans	[2],[3]	205	266
Allowance for credit losses on TDR Loans	[4]	16	12
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	96	175
Unpaid Principal Balance of TDR Loans	[2],[3]	101	180
Allowance for credit losses on TDR Loans	[4]	0	19
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	706	720
Unpaid Principal Balance of TDR Loans	[2],[3]	832	849
Allowance for credit losses on TDR Loans	[4]	7	9
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	668	683
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	762	779
Allowance for credit losses on TDR Loans	[4]	5	7
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	34	33
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	66	66
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]	4	4
Unpaid Principal Balance of TDR Loans	[2],[3]	4	4
Allowance for credit losses on TDR Loans	[4]	$ 1	$ 1

[1] At March 31, 2018 and December 31, 2017, the carrying value of TDR Loans includes $469 million and $559 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 $368 million and $329 million at March 31, 2018 and December 31, 2017, respectively.

[3] The carrying value of TDR Loans includes basis adjustments on the loans, such as partial charge-offs, unamortized deferred fees and costs on originated loans 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 TDR Loans totaled $161 million and $245 million at March 31, 2018 and December 31, 2017, respectively.

[6] At March 31, 2018 and December 31, 2017, the carrying value of consumer mortgage TDR Loans held for investment includes $644 million and $655 million, respectively, of loans that are recorded at the lower of amortized cost or fair value of the collateral less cost to sell.