Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 958	$ 1,089
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	386	383
Amount without Impairment Reserves	[4]	234	315
Total Impaired Commercial Loans	[4],[5]	620	698
Impairment Reserve		102	127
Unpaid principal balance of impaired commercial loans		732	842
TDR Loans	[2],[3],[6]	252	369
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	2	0
Amount without Impairment Reserves	[4]	10	11
Total Impaired Commercial Loans	[4],[5]	12	11
Impairment Reserve		1	0
Unpaid principal balance of impaired commercial loans		12	11
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	196	121
Amount without Impairment Reserves	[4]	90	129
Total Impaired Commercial Loans	[4],[5]	286	250
Impairment Reserve		39	45
Unpaid principal balance of impaired commercial loans		329	311
TDR Loans	[2],[3]	156	194
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	188	262
Amount without Impairment Reserves	[4]	134	175
Total Impaired Commercial Loans	[4],[5]	322	437
Impairment Reserve		62	82
Unpaid principal balance of impaired commercial loans		391	520
TDR Loans	[2],[3]	$ 96	$ 175

[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] Reflects the carrying value of impaired commercial loans and 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.

[5] Includes impaired commercial loans that are also considered TDR Loans which totaled $252 million and $369 million at March 31, 2018 and December 31, 2017, respectively.

[6] 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.