Loans - Credit Risk Profile of 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
Investment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,141
2019	3,797
2018	2,962
2017	1,584
2016	828
Prior	6,548
Revolving Loans	19,828
Revolving Loans Converted to Term Loans	91
Total loans	36,779	29,991
Investment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	287
2019	1,392
2018	1,500
2017	515
2016	202
Prior	1,205
Revolving Loans	6
Revolving Loans Converted to Term Loans	4
Total loans	5,111	6,332
Investment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	459
2019	611
2018	202
2017	104
2016	389
Prior	1,744
Revolving Loans	5,219
Revolving Loans Converted to Term Loans	87
Total loans	8,815	6,029
Investment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	319
2019	1,363
2018	1,013
2017	802
2016	141
Prior	3,299
Revolving Loans	10,386
Revolving Loans Converted to Term Loans	0
Total loans	17,323	12,981
Investment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	76
2019	431
2018	247
2017	163
2016	96
Prior	300
Revolving Loans	4,217
Revolving Loans Converted to Term Loans	0
Total loans	5,530	4,649
Noninvestment Grade [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	398
2019	3,837
2018	2,822
2017	1,247
2016	779
Prior	3,888
Revolving Loans	11,213
Revolving Loans Converted to Term Loans	283
Total loans	24,467	18,220
Noninvestment Grade [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	118
2019	2,408
2018	1,595
2017	874
2016	134
Prior	966
Revolving Loans	141
Revolving Loans Converted to Term Loans	22
Total loans	6,258	5,169
Noninvestment Grade [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	160
2019	914
2018	651
2017	311
2016	151
Prior	1,700
Revolving Loans	6,657
Revolving Loans Converted to Term Loans	261
Total loans	10,805	7,450
Noninvestment Grade [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	59
2019	469
2018	488
2017	40
2016	442
Prior	1,138
Revolving Loans	4,182
Revolving Loans Converted to Term Loans	0
Total loans	6,818	4,934
Noninvestment Grade [Member] | Other Commercial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	61
2019	46
2018	88
2017	22
2016	52
Prior	84
Revolving Loans	233
Revolving Loans Converted to Term Loans	0
Total loans	$ 586	$ 667