Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Financial Assets:
Cash and due from banks	$ 3,598	$ 3,212
Interest-bearing deposits with banks	62,324	73,040
Securities purchased under resale agreements	3,041	4,679
Investment securities held-to-maturity	39,535	41,351
Financial Liabilities:
Non-interest-bearing	33,719	44,804
Interest-bearing - U.S.	72,260	66,235
Interest-bearing - non-U.S.	64,907	69,321
Securities sold under repurchase agreements	1,330	1,082
Other short-term borrowings	7,073	3,092
Reported Amount
Financial Assets:
Cash and due from banks	3,598	3,212
Interest-bearing deposits with banks	62,324	73,040
Securities purchased under resale agreements	3,041	4,679
Investment securities held-to-maturity	39,119	41,914
Net loans (excluding leases)	26,938	25,722
Other	8,499	8,500
Financial Liabilities:
Non-interest-bearing	33,719	44,804
Interest-bearing - U.S.	72,260	66,235
Interest-bearing - non-U.S.	64,907	69,321
Securities sold under repurchase agreements	1,330	1,082
Other short-term borrowings	7,073	3,092
Long-term debt	11,455	11,093
Other	8,499	8,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	3,598	3,212
Interest-bearing deposits with banks	62,324	73,040
Securities purchased under resale agreements	3,041	4,679
Investment securities held-to-maturity	39,535	41,351
Net loans (excluding leases)	27,133	25,561
Other	8,499	8,500
Financial Liabilities:
Non-interest-bearing	33,719	44,804
Interest-bearing - U.S.	72,260	66,235
Interest-bearing - non-U.S.	64,907	69,321
Securities sold under repurchase agreements	1,330	1,082
Other short-term borrowings	7,073	3,092
Long-term debt	11,506	11,048
Other	8,499	8,500
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,598	3,212
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	11,753	14,541
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	62,324	73,040
Securities purchased under resale agreements	3,041	4,679
Investment securities held-to-maturity	27,681	26,688
Net loans (excluding leases)	25,546	24,648
Other	8,499	8,500
Financial Liabilities:
Non-interest-bearing	33,719	44,804
Interest-bearing - U.S.	72,260	66,235
Interest-bearing - non-U.S.	64,907	69,321
Securities sold under repurchase agreements	1,330	1,082
Other short-term borrowings	7,073	3,092
Long-term debt	11,345	10,865
Other	8,499	8,500
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	101	122
Net loans (excluding leases)	1,587	913
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	161	183
Other	0	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 37	$ 10