Loans - Impaired Commercial Loans Considered as TDR Loans (Detail) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Impaired [Line Items]
TDR Loans	[1],[2],[3]	$ 720	$ 859
Commercial Loans [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	91	61
Amount without Impairment Reserves	[4]	177	174
Total Impaired Commercial Loans	[4],[5]	268	235
Impairment Reserve		38	19
Unpaid principal balance of impaired commercial loans		297	256
TDR Loans	[2],[3],[6]	104	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]	3	3
Total Impaired Commercial Loans	[4],[5]	5	6
Impairment Reserve		0	1
Unpaid principal balance of impaired commercial loans		5	6
Commercial Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	57	58
Amount without Impairment Reserves	[4]	33	37
Total Impaired Commercial Loans	[4],[5]	90	95
Impairment Reserve		23	18
Unpaid principal balance of impaired commercial loans		109	109
TDR Loans	[2],[3]	36	68
Commercial Loans [Member] | Global Banking [Member]
Financing Receivable, Impaired [Line Items]
Amount with Impairment Reserves	[4]	32	0
Amount without Impairment Reserves	[4]	141	133
Total Impaired Commercial Loans	[4],[5]	173	133
Impairment Reserve		15	0
Unpaid principal balance of impaired commercial loans		183	140
TDR Loans	[2],[3]	$ 68	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 December 31, 2019 and 2018, the carrying value of TDR Loans includes $230 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 $164 million and $54 million at December 31, 2019 and 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 $104 million and $181 million at December 31, 2019 and 2018, respectively.

[6] Additional commitments to lend to commercial borrowers whose loans have been modified in TDR Loans totaled $222 million and $151 million at December 31, 2019 and 2018, respectively.