Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 942	$ 1,089
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	168	383
Amount without Impairment Reserves	[4]	273	315
Total Impaired Commercial Loans	[4],[5]	441	698
Impairment Reserve		43	127
Unpaid principal balance of impaired commercial loans		479	842
TDR Loans	[2],[3],[6]	241	369
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	3	0
Amount without Impairment Reserves	[4]	8	11
Total Impaired Commercial Loans	[4],[5]	11	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]	137	121
Amount without Impairment Reserves	[4]	120	129
Total Impaired Commercial Loans	[4],[5]	257	250
Impairment Reserve		27	45
Unpaid principal balance of impaired commercial loans		288	311
TDR Loans	[2],[3]	117	194
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	28	262
Amount without Impairment Reserves	[4]	145	175
Total Impaired Commercial Loans	[4],[5]	173	437
Impairment Reserve		15	82
Unpaid principal balance of impaired commercial loans		179	520
TDR Loans	[2],[3]	$ 124	$ 175

[1] At June 30, 2018 and December 31, 2017, the carrying value of TDR Loans includes $433 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 $200 million and $329 million at June 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] 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 $241 million and $369 million at June 30, 2018 and December 31, 2017, respectively.

[6] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $121 million and $245 million at June 30, 2018 and December 31, 2017, respectively.