Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Financial Assets:
Cash and due from banks	$ 3,886	$ 2,107	$ 3,156	$ 1,314
Interest-bearing deposits with banks	76,366	67,227
Securities purchased under resale agreements	3,583	3,241
Investment securities held-to-maturity	38,805	40,255
Financial Liabilities:
Non-interest-bearing	52,316	47,175
Interest-bearing—U.S.	57,407	50,139
Interest-bearing—non-U.S.	76,940	87,582
Securities sold under repurchase agreements	3,088	2,842
Other short-term borrowings	1,103	1,144
Reported Amount
Financial Assets:
Cash and due from banks	3,886	2,107
Interest-bearing deposits with banks	76,366	67,227
Securities purchased under resale agreements	3,583	3,241
Investment securities held-to-maturity	39,594	40,458
Net loans (excluding leases)	23,638	22,577
Other(2)	7,000
Financial Liabilities:
Non-interest-bearing	52,316	47,175
Interest-bearing—U.S.	57,407	50,139
Interest-bearing—non-U.S.	76,940	87,582
Securities sold under repurchase agreements	3,088	2,842
Other short-term borrowings	1,103	1,144
Long-term debt	10,387	11,620
Other(2)	7,000
Estimated Fair Value
Financial Assets:
Cash and due from banks	3,886	2,107
Interest-bearing deposits with banks	76,366	67,227
Securities purchased under resale agreements	3,583	3,241
Investment securities held-to-maturity	38,805	40,255
Net loans (excluding leases)	23,613	22,482
Other(2)	7,000
Financial Liabilities:
Non-interest-bearing	52,316	47,175
Interest-bearing—U.S.	57,407	50,139
Interest-bearing—non-U.S.	76,940	87,582
Securities sold under repurchase agreements	3,088	2,842
Other short-term borrowings	1,103	1,144
Long-term debt	10,597	11,919
Other(2)	7,000
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,886	2,107
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	15,639	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	76,366	67,227
Securities purchased under resale agreements	3,583	3,241
Investment securities held-to-maturity	23,043	23,318
Net loans (excluding leases)	23,571	22,431
Other(2)	7,000
Financial Liabilities:
Non-interest-bearing	52,316	47,175
Interest-bearing—U.S.	57,407	50,139
Interest-bearing—non-U.S.	76,940	87,582
Securities sold under repurchase agreements	3,088	2,842
Other short-term borrowings	1,103	1,144
Long-term debt	10,346	11,639
Other(2)	7,000
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	123	123
Net loans (excluding leases)	42	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	251	280
Other(2)	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 22	$ 3