Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Financial Assets:
Cash and due from banks	$ 4,145	$ 2,107	$ 3,939	$ 1,314
Interest-bearing deposits with banks	63,618	67,227
Securities purchased under resale agreements	4,195	3,241
Investment securities held-to-maturity	39,591	40,255
Financial Liabilities:
Non-interest-bearing	41,893	47,175
Interest-bearing - U.S.	63,661	50,139
Interest-bearing - non-U.S.	62,644	87,582
Securities sold under repurchase agreements	1,690	2,842
Other short-term borrowings	1,009	1,144
Reported Amount
Financial Assets:
Cash and due from banks	4,145	2,107
Interest-bearing deposits with banks	63,618	67,227
Securities purchased under resale agreements	4,195	3,241
Investment securities held-to-maturity	40,567	40,458
Net loans (excluding leases)	23,062	22,577
Other(2)	8,505
Financial Liabilities:
Non-interest-bearing	41,893	47,175
Interest-bearing - U.S.	63,661	50,139
Interest-bearing - non-U.S.	62,644	87,582
Securities sold under repurchase agreements	1,690	2,842
Other short-term borrowings	1,009	1,144
Long-term debt	10,335	11,620
Other(2)	8,505
Estimated Fair Value
Financial Assets:
Cash and due from banks	4,145	2,107
Interest-bearing deposits with banks	63,618	67,227
Securities purchased under resale agreements	4,195	3,241
Investment securities held-to-maturity	39,591	40,255
Net loans (excluding leases)	23,058	22,482
Other(2)	8,505
Financial Liabilities:
Non-interest-bearing	41,893	47,175
Interest-bearing - U.S.	63,661	50,139
Interest-bearing - non-U.S.	62,644	87,582
Securities sold under repurchase agreements	1,690	2,842
Other short-term borrowings	1,009	1,144
Long-term debt	10,549	11,919
Other(2)	8,505
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	4,145	2,107
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	14,785	16,814
Net loans (excluding leases)	0	0
Other(2)	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(2)	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	63,618	67,227
Securities purchased under resale agreements	4,195	3,241
Investment securities held-to-maturity	24,623	23,318
Net loans (excluding leases)	23,018	22,431
Other(2)	8,505
Financial Liabilities:
Non-interest-bearing	41,893	47,175
Interest-bearing - U.S.	63,661	50,139
Interest-bearing - non-U.S.	62,644	87,582
Securities sold under repurchase agreements	1,690	2,842
Other short-term borrowings	1,009	1,144
Long-term debt	10,309	11,639
Other(2)	8,505
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	183	123
Net loans (excluding leases)	40	51
Other(2)	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	240	280
Other(2)	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 33	$ 3