Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial Assets:
Cash and due from banks	$ 2,546	$ 2,107	$ 2,909	$ 1,314
Interest-bearing deposits with banks	79,418	67,227
Securities purchased under resale agreements	5,136	3,241
Investment securities held-to-maturity	40,483	40,255
Financial Liabilities:
Non-interest-bearing	57,025	47,175
Interest-bearing—U.S.	55,094	50,139
Interest-bearing—non-U.S.	79,398	87,582
Securities sold under repurchase agreements	2,020	2,842
Other short-term borrowings	1,066	1,144
Reported Amount
Financial Assets:
Cash and due from banks	2,546	2,107
Interest-bearing deposits with banks	79,418	67,227
Securities purchased under resale agreements	5,136	3,241
Investment securities held-to-maturity	41,158	40,458
Net loans (excluding leases)	28,864	22,577
Other	5,750
Financial Liabilities:
Non-interest-bearing	57,025	47,175
Interest-bearing—U.S.	55,094	50,139
Interest-bearing—non-U.S.	79,398	87,582
Securities sold under repurchase agreements	2,020	2,842
Other short-term borrowings	1,066	1,144
Long-term debt	10,944	11,620
Other	5,750
Estimated Fair Value
Financial Assets:
Cash and due from banks	2,546	2,107
Interest-bearing deposits with banks	79,418	67,227
Securities purchased under resale agreements	5,136	3,241
Investment securities held-to-maturity	40,483	40,255
Net loans (excluding leases)	28,859	22,482
Other	5,750
Financial Liabilities:
Non-interest-bearing	57,025	47,175
Interest-bearing—U.S.	55,094	50,139
Interest-bearing—non-U.S.	79,398	87,582
Securities sold under repurchase agreements	2,020	2,842
Other short-term borrowings	1,066	1,144
Long-term debt	11,170	11,919
Other	5,750
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	2,546	2,107
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	16,577	16,814
Net loans (excluding leases)	0	0
Other	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
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	79,418	67,227
Securities purchased under resale agreements	5,136	3,241
Investment securities held-to-maturity	23,783	23,318
Net loans (excluding leases)	28,813	22,431
Other	5,750
Financial Liabilities:
Non-interest-bearing	57,025	47,175
Interest-bearing—U.S.	55,094	50,139
Interest-bearing—non-U.S.	79,398	87,582
Securities sold under repurchase agreements	2,020	2,842
Other short-term borrowings	1,066	1,144
Long-term debt	10,909	11,639
Other	5,750
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)	46	51
Other	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	261	280
Other	0
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 10	$ 3