Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financial Assets:
Cash and due from banks	$ 4,932	$ 3,302
Interest-bearing deposits with banks	147,735	68,965
Securities purchased under resale agreements	1,037	1,487
Investment securities held-to-maturity	42,201	42,157
Financial Liabilities:
Non-interest-bearing	69,404	34,031
Interest-bearing - U.S.	110,106	77,504
Interest-bearing - non-U.S.	77,594	70,337
Securities sold under repurchase agreements	5,373	1,102
Other short-term borrowings	4,835	839
Reported Amount
Financial Assets:
Cash and due from banks	4,932	3,302
Interest-bearing deposits with banks	147,735	68,965
Securities purchased under resale agreements	1,037	1,487
Investment securities held-to-maturity	41,150	41,782
Net loans (excluding leases)	32,282	26,235
Other	4,500	7,500
Financial Liabilities:
Non-interest-bearing	69,404	34,031
Interest-bearing - U.S.	110,106	77,504
Interest-bearing - non-U.S.	77,594	70,337
Securities sold under repurchase agreements	5,373	1,102
Other short-term borrowings	4,835	839
Long-term debt	15,538	12,509
Other	4,500	7,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	4,932	3,302
Interest-bearing deposits with banks	147,735	68,965
Securities purchased under resale agreements	1,037	1,487
Investment securities held-to-maturity	42,201	42,157
Net loans (excluding leases)	31,962	26,292
Other	4,500	7,500
Financial Liabilities:
Non-interest-bearing	69,404	34,031
Interest-bearing - U.S.	110,106	77,504
Interest-bearing - non-U.S.	77,594	70,337
Securities sold under repurchase agreements	5,373	1,102
Other short-term borrowings	4,835	839
Long-term debt	14,946	12,770
Other	4,500	7,500
Estimated Fair Value | Nonrecurring Measurement Basis
Financial Assets:
Net loans (excluding leases)	2	9
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	4,932	3,302
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	9,408	10,299
Net loans (excluding leases)	0	0
Other	0	0
Financial Liabilities:
Non-interest-bearing	0	0
Interest-bearing - U.S.	0	0
Interest-bearing - non-U.S.	0	0
Securities sold under repurchase agreements	0	0
Other short-term borrowings	0	0
Long-term debt	0	0
Other	0	0
Estimated Fair Value | Pricing Methods with Significant Observable Market Inputs (Level 2)
Financial Assets:
Cash and due from banks	0	0
Interest-bearing deposits with banks	147,735	68,965
Securities purchased under resale agreements	1,037	1,487
Investment securities held-to-maturity	32,562	31,682
Net loans (excluding leases)	30,013	24,432
Other	4,500	7,500
Financial Liabilities:
Non-interest-bearing	69,404	34,031
Interest-bearing - U.S.	110,106	77,504
Interest-bearing - non-U.S.	77,594	70,337
Securities sold under repurchase agreements	5,373	1,102
Other short-term borrowings	4,835	839
Long-term debt	14,805	12,621
Other	4,500	7,500
Estimated Fair Value | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Financial Assets:
Cash and due from banks	0	0
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	231	176
Net loans (excluding leases)	1,949	1,860
Other	0	0
Financial Liabilities:
Non-interest-bearing	0	0
Interest-bearing - U.S.	0	0
Interest-bearing - non-U.S.	0	0
Securities sold under repurchase agreements	0	0
Other short-term borrowings	0	0
Long-term debt	141	149
Other	0	$ 0
Held to Maturity Under MMLF | Reported Amount
Financial Assets:
Investment securities held-to-maturity	26,808
Held to Maturity Under MMLF | Estimated Fair Value
Financial Assets:
Investment securities held-to-maturity	26,808
Held to Maturity Under MMLF | Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Investment securities held-to-maturity	0
Held to Maturity Under MMLF | Estimated Fair Value | Pricing Methods with Significant Observable Market Inputs (Level 2)
Financial Assets:
Investment securities held-to-maturity	26,808
Held to Maturity Under MMLF | Estimated Fair Value | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Financial Assets:
Investment securities held-to-maturity	0
Short Term Borrowings Under MMLF | Reported Amount
Financial Liabilities:
Other short-term borrowings	25,665
Short Term Borrowings Under MMLF | Estimated Fair Value
Financial Liabilities:
Other short-term borrowings	25,665
Short Term Borrowings Under MMLF | Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Liabilities:
Other short-term borrowings	0
Short Term Borrowings Under MMLF | Estimated Fair Value | Pricing Methods with Significant Observable Market Inputs (Level 2)
Financial Liabilities:
Other short-term borrowings	25,665
Short Term Borrowings Under MMLF | Estimated Fair Value | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Financial Liabilities:
Other short-term borrowings	$ 0