Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 874	$ 1,089
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	64	383
Amount without Impairment Reserves	[4]	186	315
Total Impaired Commercial Loans	[4],[5]	250	698
Impairment Reserve		25	127
Unpaid principal balance of impaired commercial loans		270	842
TDR Loans	[2],[3],[6]	186	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]	2	11
Total Impaired Commercial Loans	[4],[5]	5	11
Impairment Reserve		1	0
Unpaid principal balance of impaired commercial loans		5	11
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	61	121
Amount without Impairment Reserves	[4]	52	129
Total Impaired Commercial Loans	[4],[5]	113	250
Impairment Reserve		24	45
Unpaid principal balance of impaired commercial loans		126	311
TDR Loans	[2],[3]	76	194
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	0	262
Amount without Impairment Reserves	[4]	131	175
Total Impaired Commercial Loans	[4],[5]	131	437
Impairment Reserve		0	82
Unpaid principal balance of impaired commercial loans		138	520
TDR Loans	[2],[3]	110	$ 175
Commercial Loans [Member] | Other Commercial [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	0
Amount without Impairment Reserves	[4]	1
Total Impaired Commercial Loans	[4],[5]	1
Impairment Reserve		0
Unpaid principal balance of impaired commercial loans		$ 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] 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 $186 million and $369 million at September 30, 2018 and December 31, 2017, respectively.

[6] 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.