Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 731	$ 859
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	123	61
Amount without Impairment Reserves	[4]	172	174
Total Impaired Commercial Loans	[4],[5]	295	235
Impairment Reserve		30	19
Unpaid principal balance of impaired commercial loans		345	256
TDR Loans	[2],[3],[6]	100	181
Commercial Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	2	3
Amount without Impairment Reserves	[4]	2	3
Total Impaired Commercial Loans	[4],[5]	4	6
Impairment Reserve		1	1
Unpaid principal balance of impaired commercial loans		4	6
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	121	58
Amount without Impairment Reserves	[4]	38	37
Total Impaired Commercial Loans	[4],[5]	159	95
Impairment Reserve		29	18
Unpaid principal balance of impaired commercial loans		200	109
TDR Loans	[2],[3]	42	68
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	0	0
Amount without Impairment Reserves	[4]	132	133
Total Impaired Commercial Loans	[4],[5]	132	133
Impairment Reserve		0	0
Unpaid principal balance of impaired commercial loans		141	140
TDR Loans	[2],[3]	$ 58	113
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, 2019 and December 31, 2018, the carrying value of TDR Loans includes $231 million and $286 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 $195 million and $54 million at September 30, 2019 and December 31, 2018, respectively.

[3] The carrying value of TDR Loans includes basis adjustments on the loans, such as partial charge-offs.

[4] Reflects the carrying value of impaired commercial loans and includes basis adjustments on the loans, such as partial charge-offs.

[5] Includes impaired commercial loans that are also considered TDR Loans which totaled $100 million and $181 million at September 30, 2019 and December 31, 2018, respectively.

[6] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $244 million and $151 million at September 30, 2019 and December 31, 2018, respectively.