Loans - Status of Loan Portfolios (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Commercial Loans [Member]
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
Commercial Loans [Member] | 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
Commercial Loans [Member] | 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
Commercial Loans [Member] | 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
Commercial Loans [Member] | 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
Commercial Loans [Member] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,539
2019	7,618
2018	5,771
2017	2,831
2016	1,587
Prior	10,367
Revolving Loans	30,905
Revolving Loans Converted to Term Loans	371
Total loans	60,989	47,973
Commercial Loans [Member] | Performing Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	405
2019	3,800
2018	3,095
2017	1,389
2016	335
Prior	2,169
Revolving Loans	147
Revolving Loans Converted to Term Loans	24
Total loans	11,364	11,495
Commercial Loans [Member] | Performing Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	619
2019	1,509
2018	840
2017	415
2016	540
Prior	3,428
Revolving Loans	11,808
Revolving Loans Converted to Term Loans	347
Total loans	19,506	13,396
Commercial Loans [Member] | Performing Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	378
2019	1,832
2018	1,501
2017	842
2016	564
Prior	4,386
Revolving Loans	14,500
Revolving Loans Converted to Term Loans	0
Total loans	24,003	17,766
Commercial Loans [Member] | Performing Loans [Member] | 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
Commercial Loans [Member] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	16
2018	13
2017	0
2016	20
Prior	69
Revolving Loans	134
Revolving Loans Converted to Term Loans	3
Total loans	255	237
Commercial Loans [Member] | Nonaccrual Loans [Member] | Real Estate, Including Construction [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017
2016	1
Prior	2
Revolving Loans	0
Revolving Loans Converted to Term Loans	2
Total loans	5	6
Commercial Loans [Member] | Nonaccrual Loans [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	16
2018	13
2017	0
2016	0
Prior	16
Revolving Loans	66
Revolving Loans Converted to Term Loans	1
Total loans	112	82
Commercial Loans [Member] | Nonaccrual Loans [Member] | Global Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	19
Prior	51
Revolving Loans	68
Revolving Loans Converted to Term Loans	0
Total loans	138	149
Commercial Loans [Member] | Nonaccrual Loans [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
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	2
Revolving Loans Converted to Term Loans	0
Total loans	2	1
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Business and Corporate Banking [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	2
Revolving Loans Converted to Term Loans	0
Total loans	2	1
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Global Banking [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
Commercial Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [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
Consumer Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,087
2019	3,093
2018	1,850
2017	2,105
2016	2,428
Prior	8,521
Revolving Loans	1,398
Total loans	20,482	20,342
Consumer Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,032
2019	2,976
2018	1,805
2017	2,062
2016	2,371
Prior	7,788
Revolving Loans	0
Total loans	18,034	17,801
Consumer Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14
2019	54
2018	44
2017	41
2016	53
Prior	608
Revolving Loans	0
Total loans	814	853
Consumer Loans [Member] | Credit Cards [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	1,300
Total loans	1,300	1,405
Consumer Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	41
2019	63
2018	1
2017	2
2016	4
Prior	125
Revolving Loans	98
Total loans	334	283
Consumer Loans [Member] | Performing Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,087
2019	3,085
2018	1,844
2017	2,092
2016	2,416
Prior	8,152
Revolving Loans	1,368
Total loans	20,044	19,886
Consumer Loans [Member] | Performing Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,032
2019	2,968
2018	1,799
2017	2,049
2016	2,359
Prior	7,457
Revolving Loans	0
Total loans	17,664	17,420
Consumer Loans [Member] | Performing Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	14
2019	54
2018	44
2017	41
2016	53
Prior	570
Revolving Loans	0
Total loans	776	807
Consumer Loans [Member] | Performing Loans [Member] | Credit Cards [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	1,272
Total loans	1,272	1,381
Consumer Loans [Member] | Performing Loans [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	41
2019	63
2018	1
2017	2
2016	4
Prior	125
Revolving Loans	96
Total loans	332	278
Consumer Loans [Member] | Nonaccrual Loans [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	8
2018	6
2017	13
2016	12
Prior	369
Revolving Loans	0
Total loans	408	427
Consumer Loans [Member] | Nonaccrual Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	8
2018	6
2017	13
2016	12
Prior	331
Revolving Loans	0
Total loans	370	381
Consumer Loans [Member] | Nonaccrual Loans [Member] | Home Equity Mortgages [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	38
Revolving Loans	0
Total loans	38	46
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	30
Total loans	30	29
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Credit Cards [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	28
Total loans	28	24
Consumer Loans [Member] | Nonperforming Loans, Accruing Loans 90 Days or More Past Due [Member] | Other consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Revolving Loans	2
Total loans	$ 2	$ 5