Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 1,089	$ 1,222
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	383	596
Amount without Impairment Reserves	[4]	315	457
Total Impaired Commercial Loans	[4],[5]	698	1,053
Impairment Reserve		127	308
Unpaid principal balance of impaired commercial loans		842	1,123
TDR Loans	[2],[3],[6]	369	482
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	0	2
Amount without Impairment Reserves	[4]	11	41
Total Impaired Commercial Loans	[4],[5]	11	43
Impairment Reserve		0	1
Unpaid principal balance of impaired commercial loans		11	45
TDR Loans	[2],[3]	0	32
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	121	176
Amount without Impairment Reserves	[4]	129	166
Total Impaired Commercial Loans	[4],[5]	250	342
Impairment Reserve		45	55
Unpaid principal balance of impaired commercial loans		311	397
TDR Loans	[2],[3]	194	300
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	262	417
Amount without Impairment Reserves	[4]	175	244
Total Impaired Commercial Loans	[4],[5]	437	661
Impairment Reserve		82	251
Unpaid principal balance of impaired commercial loans		520	674
TDR Loans	[2],[3]	175	150
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	0	1
Amount without Impairment Reserves	[4]	0	6
Total Impaired Commercial Loans	[4],[5]	0	7
Impairment Reserve		0	1
Unpaid principal balance of impaired commercial loans		$ 0	$ 7

[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] 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 any premiums or discounts on purchased loans.

[5] Includes impaired commercial loans that are also considered TDR Loans which totaled $369 million and $482 million at December 31, 2017 and 2016, respectively.

[6] 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.