Loans - Summary of Criticized Assets for Commercial Loans (Detail) - Commercial Loans [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
2020	$ 1,539
2019	7,634
2018	5,784
2017	2,831
2016	1,607
Prior	10,436
Revolving Loans	31,041
Revolving Loans Converted to Term Loans	374
Total loans	61,246	$ 48,211
Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	405
2019	3,800
2018	3,095
2017	1,389
2016	336
Prior	2,171
Revolving Loans	147
Revolving Loans Converted to Term Loans	26
Total loans	11,369	11,501
Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	619
2019	1,525
2018	853
2017	415
2016	540
Prior	3,444
Revolving Loans	11,876
Revolving Loans Converted to Term Loans	348
Total loans	19,620	13,479
Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	378
2019	1,832
2018	1,501
2017	842
2016	583
Prior	4,437
Revolving Loans	14,568
Revolving Loans Converted to Term Loans	0
Total loans	24,141	17,915
Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	137
2019	477
2018	335
2017	185
2016	148
Prior	384
Revolving Loans	4,450
Revolving Loans Converted to Term Loans	0
Total loans	6,116	5,316
Criticized [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	664
2018	226
2017	196
2016	18
Prior	803
Revolving Loans	1,651
Revolving Loans Converted to Term Loans	28
Total loans	3,586	2,001
Criticized [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	532
2018	157
2017	122
2016	0
Prior	279
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	1,090	719
Criticized [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	70
2018	58
2017	74
2016	15
Prior	303
Revolving Loans	986
Revolving Loans Converted to Term Loans	28
Total loans	1,534	876
Criticized [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	38
2018	0
2017	0
2016	0
Prior	201
Revolving Loans	604
Revolving Loans Converted to Term Loans	0
Total loans	843	395
Criticized [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	24
2018	11
2017	0
2016	3
Prior	20
Revolving Loans	61
Revolving Loans Converted to Term Loans	0
Total loans	119	11
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	489
2018	182
2017	172
2016	16
Prior	520
Revolving Loans	889
Revolving Loans Converted to Term Loans	24
Total loans	2,292	1,178
Special Mention [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	402
2018	150
2017	122
2016	0
Prior	234
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	908	516
Special Mention [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	25
2018	21
2017	50
2016	13
Prior	164
Revolving Loans	555
Revolving Loans Converted to Term Loans	24
Total loans	852	467
Special Mention [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	38
2018	0
2017	0
2016	0
Prior	102
Revolving Loans	334
Revolving Loans Converted to Term Loans	0
Total loans	474	184
Special Mention [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	24
2018	11
2017	0
2016	3
Prior	20
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	58	11
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	159
2018	31
2017	24
2016	2
Prior	252
Revolving Loans	698
Revolving Loans Converted to Term Loans	2
Total loans	1,168	785
Substandard [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	130
2018	7
2017	0
2016	0
Prior	45
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	182	203
Substandard [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	29
2018	24
2017	24
2016	2
Prior	121
Revolving Loans	367
Revolving Loans Converted to Term Loans	2
Total loans	569	386
Substandard [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	86
Revolving Loans	270
Revolving Loans Converted to Term Loans	0
Total loans	356	196
Substandard [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	61
Revolving Loans Converted to Term Loans	0
Total loans	61	0
Doubtful [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	16
2018	13
2017	0
2016	0
Prior	31
Revolving Loans	64
Revolving Loans Converted to Term Loans	2
Total loans	126	38
Doubtful [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	0	0
Doubtful [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	16
2018	13
2017	0
2016	0
Prior	18
Revolving Loans	64
Revolving Loans Converted to Term Loans	2
Total loans	113	23
Doubtful [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	13
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	13	15
Doubtful [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	0
Revolving Loans Converted to Term Loans	0
Total loans	$ 0	$ 0