Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 874	$ 1,089
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,022	1,295
Allowance for credit losses on TDR Loans	[4]	11	40
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	186	369
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	211	446
Allowance for credit losses on TDR Loans	[4]	6	31
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	76	194
Unpaid Principal Balance of TDR Loans	[2],[3]	95	266
Allowance for credit losses on TDR Loans	[4]	6	12
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	110	175
Unpaid Principal Balance of TDR Loans	[2],[3]	116	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]	688	720
Unpaid Principal Balance of TDR Loans	[2],[3]	811	849
Allowance for credit losses on TDR Loans	[4]	5	9
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	649	683
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	740	779
Allowance for credit losses on TDR Loans	[4]	3	7
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	35	33
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	67	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 September 30, 2018 and December 31, 2017, the carrying value of TDR Loans includes $340 million and $559 million, respectively, of loans which are classified as nonaccrual.

[2] TDR Loans are considered to be impaired loans. For commercial loans, impaired loans include other loans in addition to TDR Loans which totaled $64 million and $329 million at September 30, 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 $105 million and $245 million at September 30, 2018 and December 31, 2017, respectively.

[6] At September 30, 2018 and December 31, 2017, the carrying value of consumer mortgage TDR Loans held for investment includes $623 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.