Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financial Assets:
Cash and due from banks	$ 4,469	$ 3,597
Interest-bearing deposits with banks	53,864	73,040
Securities purchased under resale agreements	1,522	4,679
Investment securities held-to-maturity	40,971	41,351
Financial Liabilities:
Non-interest-bearing	35,295	44,804
Interest-bearing - U.S.	62,988	66,235
Interest-bearing - non-U.S.	64,188	69,321
Securities sold under repurchase agreements	1,420	1,082
Other short-term borrowings	947	3,092
Reported Amount
Financial Assets:
Cash and due from banks	4,469	3,597
Interest-bearing deposits with banks	53,864	73,040
Securities purchased under resale agreements	1,522	4,679
Investment securities held-to-maturity	41,145	41,914
Net loans (excluding leases)	23,311	25,722
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing	35,295	44,804
Interest-bearing - U.S.	62,988	66,235
Interest-bearing - non-U.S.	64,188	69,321
Securities sold under repurchase agreements	1,420	1,082
Other short-term borrowings	947	3,092
Long-term debt	11,182	11,093
Other	8,500	8,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	4,469	3,597
Interest-bearing deposits with banks	53,864	73,040
Securities purchased under resale agreements	1,522	4,679
Investment securities held-to-maturity	40,971	41,351
Net loans (excluding leases)	23,288	25,561
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing	35,295	44,804
Interest-bearing - U.S.	62,988	66,235
Interest-bearing - non-U.S.	64,188	69,321
Securities sold under repurchase agreements	1,420	1,082
Other short-term borrowings	947	3,092
Long-term debt	11,319	11,048
Other	8,500	8,500
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	4,469	3,597
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	13,198	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	53,864	73,040
Securities purchased under resale agreements	1,522	4,679
Investment securities held-to-maturity	27,652	26,688
Net loans (excluding leases)	22,245	24,648
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing	35,295	44,804
Interest-bearing - U.S.	62,988	66,235
Interest-bearing - non-U.S.	64,188	69,321
Securities sold under repurchase agreements	1,420	1,082
Other short-term borrowings	947	3,092
Long-term debt	11,142	10,865
Other	8,500	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	121	122
Net loans (excluding leases)	1,043	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	177	183
Other	$ 0	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)		$ 10