Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial Assets:
Cash and due from banks	$ 3,742	$ 3,597
Interest-bearing deposits with banks	62,534	73,040
Securities purchased under resale agreements	1,732	4,679
Investment securities held-to-maturity	39,473	41,351
Financial Liabilities:
Non-interest-bearing	34,278	44,804
Interest-bearing - U.S.	68,964	66,235
Interest-bearing - non-U.S.	67,352	69,321
Securities sold under repurchase agreements	1,829	1,082
Other short-term borrowings	4,939	3,092
Reported Amount
Financial Assets:
Cash and due from banks		3,597
Interest-bearing deposits with banks		73,040
Securities purchased under resale agreements		4,679
Investment securities held-to-maturity	39,236	41,914
Net loans (excluding leases)	25,349	25,722
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing		44,804
Interest-bearing - U.S.		66,235
Interest-bearing - non-U.S.		69,321
Securities sold under repurchase agreements		1,082
Other short-term borrowings		3,092
Long-term debt	11,374	11,093
Other	8,500	8,500
Estimated Fair Value
Financial Assets:
Cash and due from banks	3,742	3,597
Interest-bearing deposits with banks	62,534	73,040
Securities purchased under resale agreements	1,732	4,679
Investment securities held-to-maturity	39,473	41,351
Net loans (excluding leases)	25,498	25,561
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing	34,278	44,804
Interest-bearing - U.S.	68,964	66,235
Interest-bearing - non-U.S.	67,352	69,321
Securities sold under repurchase agreements	1,829	1,082
Other short-term borrowings	4,939	3,092
Long-term debt	11,430	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	3,742	3,597
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	12,378	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	62,534	73,040
Securities purchased under resale agreements	1,732	4,679
Investment securities held-to-maturity	26,995	26,688
Net loans (excluding leases)	24,113	24,648
Other	8,500	8,500
Financial Liabilities:
Non-interest-bearing	34,278	44,804
Interest-bearing - U.S.	68,964	66,235
Interest-bearing - non-U.S.	67,352	69,321
Securities sold under repurchase agreements	1,829	1,082
Other short-term borrowings	4,939	3,092
Long-term debt	11,262	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	100	122
Net loans (excluding leases)	1,385	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	168	183
Other	$ 0	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)		$ 10