Fair Value - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 839	$ 860
Investment securities available-for-sale	54,757	45,148
Derivative assets	5,040	5,189
Derivative asset, collateral, cash offset	793	987
Derivative liability, collateral, cash offset	1,432	1,341
Non-US debt securities, supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	4,290	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	730	1,300
Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,520	1,330
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	839	860
Investment securities available-for-sale	54,757	45,148
Derivative asset, Impact of Netting	(8,894)	(11,210)
Derivative assets	5,040	5,189
Other	424	395
Other assets - impact of netting	0	0
Total, derivative asset	61,060	51,592
Derivative liability, Impact of Netting	(9,532)	(11,564)
Derivative liabilities	4,915	5,243
Total liabilities carried at fair value	4,919	5,243
Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	(8,886)	(11,210)
Derivative assets	5,026	5,176
Derivative liability, Impact of Netting	(9,524)	(11,564)
Derivative liabilities	4,665	4,958
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	(8)	0
Derivative assets	14	13
Derivative liability, Impact of Netting	(8)	0
Derivative liabilities	43	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	207	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	345	325
Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	460	501
Investment securities available-for-sale	2,465	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	2,039	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	22,379	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	24,418	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	471	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	90	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	1,446	593
Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	2,007	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	1,917	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	1,831	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	13,189	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	6,998	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	23,935	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	1,819	1,918
Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	113	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	191	180
Investment securities available-for-sale	2,039	1,039
Derivative asset	0	13
Other	0	0
Total, derivative asset	2,230	1,232
Derivative liability	6	0
Total liabilities carried at fair value	10	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	4	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	2	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	137	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	20	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	2,039	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	2,039	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	648	680
Investment securities available-for-sale	50,565	42,825
Derivative asset	13,920	16,382
Other	424	395
Total, derivative asset	65,557	60,282
Derivative liability	14,429	16,803
Total liabilities carried at fair value	14,429	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	13,898	16,382
Derivative liability	14,173	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	22	0
Derivative liability	49	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	207	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	208	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	440	501
Investment securities available-for-sale	2,465	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	22,379	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	22,379	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	471	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	90	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	58	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	619	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	1,917	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	1,110	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	13,189	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	6,954	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	23,170	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	1,819	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	113	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	2,153	1,284
Derivative asset	14	4
Other	0	0
Total, derivative asset	2,167	1,288
Derivative liability	12	4
Total liabilities carried at fair value	12	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	14	4
Derivative liability	12	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	1,388	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	1,388	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	721	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	44	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	765	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	4
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	4
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