Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financial Assets:
Cash and due from banks	$ 4,848	$ 3,302
Interest-bearing deposits with banks	75,967	68,965
Securities purchased under resale agreements	4,499	1,487
Investment securities held-to-maturity	46,510	42,157
Financial Liabilities:
Non-interest-bearing	41,183	34,031
Interest-bearing - U.S.	85,434	77,504
Interest-bearing - non-U.S.	70,896	70,337
Securities sold under repurchase agreements	2,430	1,102
Other short-term borrowings	5,838	839
Long-term debt
Reported Amount
Financial Assets:
Cash and due from banks	4,848	3,302
Interest-bearing deposits with banks	75,967	68,965
Securities purchased under resale agreements	4,499	1,487
Investment securities held-to-maturity	45,394	41,782
Net loans (excluding leases)	26,901	26,235
Other	3,250	7,500
Financial Liabilities:
Non-interest-bearing	41,183	34,031
Interest-bearing - U.S.	85,434	77,504
Interest-bearing - non-U.S.	70,896	70,337
Securities sold under repurchase agreements	2,430	1,102
Other short-term borrowings	5,838	839
Long-term debt	13,853	12,509
Other	3,250	7,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	4,848	3,302
Interest-bearing deposits with banks	75,967	68,965
Securities purchased under resale agreements	4,499	1,487
Investment securities held-to-maturity	46,510	42,157
Net loans (excluding leases)	26,891	26,292
Other	3,250	7,500
Financial Liabilities:
Non-interest-bearing	41,183	34,031
Interest-bearing - U.S.	85,434	77,504
Interest-bearing - non-U.S.	70,896	70,337
Securities sold under repurchase agreements	2,430	1,102
Other short-term borrowings	5,838	839
Long-term debt	14,170	12,770
Other	3,250	7,500
Estimated Fair Value | Nonrecurring Measurement Basis
Financial Assets:
Net loans (excluding leases)	8	9
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	4,848	3,302
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	6,751	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	75,967	68,965
Securities purchased under resale agreements	4,499	1,487
Investment securities held-to-maturity	39,352	31,682
Net loans (excluding leases)	24,846	24,432
Other	3,250	7,500
Financial Liabilities:
Non-interest-bearing	41,183	34,031
Interest-bearing - U.S.	85,434	77,504
Interest-bearing - non-U.S.	70,896	70,337
Securities sold under repurchase agreements	2,430	1,102
Other short-term borrowings	5,838	839
Long-term debt	14,047	12,621
Other	3,250	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	407	176
Net loans (excluding leases)	2,045	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	123	149
Other	0	$ 0
Held to Maturity Under MMLF | Reported Amount
Financial Assets:
Investment securities held-to-maturity	4,824
Held to Maturity Under MMLF | Estimated Fair Value
Financial Assets:
Investment securities held-to-maturity	4,841
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	4,841
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	4,819
Short Term Borrowings Under MMLF | Estimated Fair Value
Financial Liabilities:
Other short-term borrowings	4,819
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	4,819
Short Term Borrowings Under MMLF | Estimated Fair Value | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Financial Liabilities:
Other short-term borrowings	$ 0