Fair Value - Schedule of Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 860	$ 1,093
Investment securities available-for-sale	45,148	57,121
Derivative asset, collateral, cash offset	987	2,045
Derivative liability, collateral, cash offset	1,341	422
Non-US debt securities, covered bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,295	3,537
Non-U.S. debt securities, other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,331	1,885
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	860	1,093
Investment securities available-for-sale	45,148	57,121
Derivative asset, Impact of Netting	(11,210)	(7,593)
Derivative assets	5,189	4,013
Other	395
Other assets - impact of netting	0
Total, derivative asset	51,592	62,227
Derivative liability, Impact of Netting	(11,564)	(5,970)
Derivative liabilities	5,243	5,882
Total liabilities carried at fair value	5,243	5,921
Recurring | Foreign exchange contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	(11,210)	(7,593)
Derivative assets	5,176	4,004
Derivative liability, Impact of Netting	(11,564)	(5,970)
Derivative liabilities	4,958	5,498
Recurring | Interest rate contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting	0	0
Derivative assets	13	8
Derivative liability, Impact of Netting	0	0
Derivative liabilities	71	100
Recurring | Other derivative contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative asset, Impact of Netting		0
Derivative assets		1
Derivative liability, Impact of Netting	0	0
Derivative liabilities	214	284
Recurring | U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	34	39
Recurring | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	325	482
Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	501	572
Investment securities available-for-sale	1,658	2,560
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	1,039	223
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	15,968	10,872
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	17,007	11,095
Recurring | Asset-backed securities, student loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	541	3,358
Recurring | Asset-backed securities, credit cards
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	583	1,542
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	593	1,447
Recurring | Total asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,717	6,347
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	1,682	6,695
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	1,574	2,947
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	12,793	10,721
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	6,602	6,108
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	22,651	26,471
Recurring | State and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,918	9,151
Recurring | Collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	197	1,054
Recurring | U.S. equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		46
Recurring | U.S. money-market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		397
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	180	472
Investment securities available-for-sale	1,039	11
Derivative asset	13	9
Other	0
Total, derivative asset	1,232	492
Derivative liability	0	1
Total liabilities carried at fair value	0	40
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	0	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	13	8
Derivative liability	0	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 asset		1
Derivative liability	0	1
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	39
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	389
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	0	44
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	1,039	11
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	1,039	11
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
Quoted Market Prices in Active Markets (Level 1) | Recurring | U.S. equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		0
Quoted Market Prices in Active Markets (Level 1) | Recurring | U.S. money-market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		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	680	621
Investment securities available-for-sale	42,825	54,984
Derivative asset	16,382	11,596
Other	395
Total, derivative asset	60,282	67,201
Derivative liability	16,803	11,850
Total liabilities carried at fair value	16,803	11,850
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	16,382	11,596
Derivative liability	16,518	11,467
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	0	0
Derivative liability	71	100
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 asset		0
Derivative liability	214	283
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	179	93
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	501	528
Investment securities available-for-sale	1,658	2,560
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	212
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	15,968	10,872
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	15,968	11,084
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	541	3,358
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	583	1,542
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	89
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	1,124	4,989
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	1,682	6,576
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	943	2,545
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	12,793	10,721
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	6,544	5,904
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	21,962	25,746
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	1,918	9,108
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	195	1,054
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | U.S. equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		46
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | U.S. money-market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		397
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	1,284	2,126
Derivative asset	4	1
Other	0
Total, derivative asset	1,288	2,127
Derivative liability	4	1
Total liabilities carried at fair value	4	1
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	1
Derivative liability	4	1
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 asset		0
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	593	1,358
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	593	1,358
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	119
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	631	402
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	58	204
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	689	725
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	43
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	$ 2	0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | U.S. equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		0
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | U.S. money-market mutual funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale		0
U.S. government securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account liabilities		39
U.S. government securities | Quoted Market Prices in Active Markets (Level 1) | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account liabilities		39
U.S. government securities | 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 liabilities		0
U.S. government securities | 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 liabilities		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 account liabilities, impact of netting		$ 0