Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financial Assets:
Cash and due from banks	$ 3,685	$ 3,302
Interest-bearing deposits with banks	90,199	68,965
Securities purchased under resale agreements	4,026	1,487
Investment securities held-to-maturity	43,037	42,157
Financial Liabilities:
Non-interest-bearing	42,132	34,031
Interest-bearing - U.S.	87,197	77,504
Interest-bearing - non-U.S.	71,133	70,337
Securities sold under repurchase agreements	3,513	1,102
Other short-term borrowings	912	839
Reported Amount
Financial Assets:
Cash and due from banks	3,685	3,302
Interest-bearing deposits with banks	90,199	68,965
Securities purchased under resale agreements	4,026	1,487
Investment securities held-to-maturity	41,848	41,782
Net loans (excluding leases)	26,719	26,235
Other	2,502	7,500
Financial Liabilities:
Non-interest-bearing	42,132	34,031
Interest-bearing - U.S.	87,197	77,504
Interest-bearing - non-U.S.	71,133	70,337
Securities sold under repurchase agreements	3,513	1,102
Other short-term borrowings	912	839
Long-term debt	15,587	12,509
Other	2,502	7,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	3,685	3,302
Interest-bearing deposits with banks	90,199	68,965
Securities purchased under resale agreements	4,026	1,487
Investment securities held-to-maturity	43,037	42,157
Net loans (excluding leases)	26,583	26,292
Other	2,502	7,500
Financial Liabilities:
Non-interest-bearing	42,132	34,031
Interest-bearing - U.S.	87,197	77,504
Interest-bearing - non-U.S.	71,133	70,337
Securities sold under repurchase agreements	3,513	1,102
Other short-term borrowings	912	839
Long-term debt	15,865	12,770
Other	2,502	7,500
Estimated Fair Value | Nonrecurring Measurement Basis
Financial Assets:
Net loans (excluding leases)	21	9
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,685	3,302
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	8,288	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	90,199	68,965
Securities purchased under resale agreements	4,026	1,487
Investment securities held-to-maturity	34,344	31,682
Net loans (excluding leases)	24,511	24,432
Other	2,502	7,500
Financial Liabilities:
Non-interest-bearing	42,132	34,031
Interest-bearing - U.S.	87,197	77,504
Interest-bearing - non-U.S.	71,133	70,337
Securities sold under repurchase agreements	3,513	1,102
Other short-term borrowings	912	839
Long-term debt	15,732	12,621
Other	2,502	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	405	176
Net loans (excluding leases)	2,072	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	133	149
Other	0	$ 0
Held to Maturity Under MMLF | Reported Amount
Financial Assets:
Investment securities held-to-maturity	11,257
Held to Maturity Under MMLF | Estimated Fair Value
Financial Assets:
Investment securities held-to-maturity	11,294
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	11,294
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	11,261
Short Term Borrowings Under MMLF | Estimated Fair Value
Financial Liabilities:
Other short-term borrowings	11,261
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	11,261
Short Term Borrowings Under MMLF | Estimated Fair Value | Pricing Methods with Significant Unobservable Market Inputs (Level 3)
Financial Liabilities:
Other short-term borrowings	$ 0