Fair Value - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 914	$ 860
Investment securities available-for-sale	53,815	45,148
Derivative assets	4,753	5,189
Derivative asset, collateral, cash offset	2,310	987
Derivative liability, collateral, cash offset	837	1,341
Non-US debt securities, supranational and non US agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	5,500	3,200
Non-US debt securities, covered bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	680	1,300
Non-U.S. debt securities, corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,780	1,330
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	914	860
Investment securities available-for-sale	53,815	45,148
Derivative asset, Impact of Netting	(10,395)	(11,210)
Derivative assets	4,753	5,189
Other	504	395
Other assets - impact of netting	0	0
Total, derivative asset	59,986	51,592
Derivative liability, Impact of Netting	(8,922)	(11,564)
Derivative liabilities	6,459	5,243
Total liabilities carried at fair value	6,464	5,243
Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	(10,391)	(11,210)
Derivative assets	4,749	5,176
Derivative liability, Impact of Netting	(8,918)	(11,564)
Derivative liabilities	6,232	4,958
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	(4)	0
Derivative assets	4	13
Derivative liability, Impact of Netting	(4)	0
Derivative liabilities	45	71
Recurring | Other derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, Impact of Netting	0	0
Derivative liabilities	182	214
Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	34	34
Recurring | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	319	325
Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	561	501
Investment securities available-for-sale	2,973	1,658
Recurring | US Treasury and federal agencies, direct obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3,487	1,039
Recurring | US Treasury and federal agencies, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	17,838	15,968
Recurring | Total U.S. Treasury and federal agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	21,325	17,007
Recurring | Asset-backed securities, student loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	531	541
Recurring | Asset-backed securities, credit cards
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	89	583
Recurring | Asset-backed securities, collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,820	593
Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	2,440	1,717
Recurring | Non-U.S. debt securities, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,980	1,682
Recurring | Non-U.S. debt securities, asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	2,179	1,574
Recurring | Non-U.S. debt securities, government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	12,373	12,793
Recurring | Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	8,658	6,602
Recurring | Total non-U.S. debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	25,190	22,651
Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,783	1,918
Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	104	197
Quoted Market Prices in Active Markets (Level 1) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	201	180
Investment securities available-for-sale	3,487	1,039
Derivative asset	0	13
Other	0	0
Total, derivative asset	3,688	1,232
Derivative liability	9	0
Total liabilities carried at fair value	14	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	3	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	13
Derivative liability	6	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Other derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	34	34
Quoted Market Prices in Active Markets (Level 1) | Recurring | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	146	146
Quoted Market Prices in Active Markets (Level 1) | Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	21	0
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | US Treasury and federal agencies, direct obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3,487	1,039
Quoted Market Prices in Active Markets (Level 1) | Recurring | US Treasury and federal agencies, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Total U.S. Treasury and federal agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3,487	1,039
Quoted Market Prices in Active Markets (Level 1) | Recurring | Asset-backed securities, student loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Asset-backed securities, credit cards
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Asset-backed securities, collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Non-U.S. debt securities, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Non-U.S. debt securities, asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Non-U.S. debt securities, government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Total non-U.S. debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	713	680
Investment securities available-for-sale	47,576	42,825
Derivative asset	15,144	16,382
Other	504	395
Total, derivative asset	63,937	60,282
Derivative liability	15,369	16,803
Total liabilities carried at fair value	15,369	16,803
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	15,136	16,382
Derivative liability	15,144	16,518
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	8	0
Derivative liability	43	71
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Other derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	182	214
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	0	0
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	173	179
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	540	501
Investment securities available-for-sale	2,973	1,658
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | US Treasury and federal agencies, direct obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | US Treasury and federal agencies, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	17,838	15,968
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Total U.S. Treasury and federal agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	17,838	15,968
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Asset-backed securities, student loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	531	541
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Asset-backed securities, credit cards
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	89	583
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Asset-backed securities, collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	620	1,124
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Non-U.S. debt securities, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,980	1,682
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Non-U.S. debt securities, asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,292	943
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Non-U.S. debt securities, government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	12,373	12,793
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	8,613	6,544
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Total non-U.S. debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	24,258	21,962
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,783	1,918
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	104	195
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	0	0
Investment securities available-for-sale	2,752	1,284
Derivative asset	4	4
Other	0	0
Total, derivative asset	2,756	1,288
Derivative liability	3	4
Total liabilities carried at fair value	3	4
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	4	4
Derivative liability	3	4
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset	0	0
Derivative liability	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Other derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	0	0
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | US Treasury and federal agencies, direct obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | US Treasury and federal agencies, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Total U.S. Treasury and federal agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Asset-backed securities, student loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Asset-backed securities, credit cards
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Asset-backed securities, collateralized loan obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,820	593
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,820	593
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Non-U.S. debt securities, mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Non-U.S. debt securities, asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	887	631
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Non-U.S. debt securities, government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	45	58
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Total non-U.S. debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	932	689
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	$ 2
Other | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading liability	5
Other | Quoted Market Prices in Active Markets (Level 1) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading liability	5
Other | Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading liability	0
Other | Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading liability	0
Trading liability, Impact of Netting	$ 0