Loans - Summary of TDR and Related Credit Loss Reserves for TDR Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[1],[2],[3]	$ 1,089	$ 1,222
Unpaid Principal Balance of TDR Loans	[1],[2],[3]	1,295	1,409
Allowance for credit losses on TDR Loans	[4]	40	48
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[5]	369	482
Unpaid Principal Balance of TDR Loans	[2],[3],[5]	446	548
Allowance for credit losses on TDR Loans	[4]	31	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	[4]	0	0
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	194	300
Unpaid Principal Balance of TDR Loans	[2],[3]	266	363
Allowance for credit losses on TDR Loans	[4]	12	37
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	175	150
Unpaid Principal Balance of TDR Loans	[2],[3]	180	152
Allowance for credit losses on TDR Loans	[4]	19	0
Consumer Loans [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3]	720	740
Unpaid Principal Balance of TDR Loans	[2],[3]	849	861
Allowance for credit losses on TDR Loans	[4]	9	11
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	683	708
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	779	797
Allowance for credit losses on TDR Loans	[4]	7	9
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Carrying Value of TDR Loans	[2],[3],[6]	33	27
Unpaid Principal Balance of TDR Loans	[2],[3],[6]	66	59
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	5
Unpaid Principal Balance of TDR Loans	[2],[3]	4	5
Allowance for credit losses on TDR Loans	[4]	$ 1	$ 1

[1] At December 31, 2017 and 2016, the carrying value of TDR Loans includes $559 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 $329 million and $571 million at December 31, 2017 and 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] Included in the allowance for credit losses.

[5] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $245 million and $184 million at December 31, 2017 and 2016, respectively.

[6] At December 31, 2017 and 2016, the carrying value of consumer mortgage TDR Loans held for investment includes $655 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.