Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 1,091	$ 1,222
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	556	596
Amount without Impairment Reserves	[4]	297	457
Total Impaired Commercial Loans	[4],[5]	853	1,053
Impairment Reserve		214	308
Unpaid principal balance of impaired commercial loans		931	1,123
TDR Loans	[2],[3],[6]	363	482
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	1	2
Amount without Impairment Reserves	[4]	16	41
Total Impaired Commercial Loans	[4],[5]	17	43
Impairment Reserve		1	1
Unpaid principal balance of impaired commercial loans		17	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]	183	176
Amount without Impairment Reserves	[4]	137	166
Total Impaired Commercial Loans	[4],[5]	320	342
Impairment Reserve		51	55
Unpaid principal balance of impaired commercial loans		384	397
TDR Loans	[2],[3]	244	300
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	372	417
Amount without Impairment Reserves	[4]	144	244
Total Impaired Commercial Loans	[4],[5]	516	661
Impairment Reserve		162	251
Unpaid principal balance of impaired commercial loans		530	674
TDR Loans	[2],[3]	119	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 September 30, 2017 and December 31, 2016, the carrying value of TDR Loans includes $514 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 $490 million and $571 million at September 30, 2017 and December 31, 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 $363 million and $482 million at September 30, 2017 and December 31, 2016, respectively.

[6] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $223 million and $184 million at September 30, 2017 and December 31, 2016, respectively.