Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Assets:
Cash and due from banks	$ 2,107	$ 1,314	$ 1,207	$ 1,855
Interest-bearing deposits with banks	67,227	70,935
Securities purchased under resale agreements	3,241	1,956
Investment securities held-to-maturity	40,255	34,994
Financial Liabilities:
Non-interest-bearing	47,175	59,397
Interest-bearing—U.S.	50,139	30,911
Interest-bearing—non-U.S.	87,582	96,855
Securities sold under repurchase agreements	2,842	4,400
Other short-term borrowings	1,144	1,585
Reported Amount
Financial Assets:
Cash and due from banks	2,107	1,314
Interest-bearing deposits with banks	67,227	70,935
Securities purchased under resale agreements	3,241	1,956
Investment securities held-to-maturity	40,458	35,169
Net loans (excluding leases)	22,577	18,862
Financial Liabilities:
Non-interest-bearing	47,175	59,397
Interest-bearing—U.S.	50,139	30,911
Interest-bearing—non-U.S.	87,582	96,855
Securities sold under repurchase agreements	2,842	4,400
Other short-term borrowings	1,144	1,585
Long-term debt	11,620	11,430
Estimated Fair Value
Financial Assets:
Cash and due from banks	2,107	1,314
Interest-bearing deposits with banks	67,227	70,935
Securities purchased under resale agreements	3,241	1,956
Investment securities held-to-maturity	40,255	34,994
Net loans (excluding leases)	22,482	18,877
Financial Liabilities:
Non-interest-bearing	47,175	59,397
Interest-bearing—U.S.	50,139	30,911
Interest-bearing—non-U.S.	87,582	96,855
Securities sold under repurchase agreements	2,842	4,400
Other short-term borrowings	1,144	1,585
Long-term debt	11,919	11,618
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	2,107	1,314
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	16,814	17,400
Net loans (excluding leases)	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
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	67,227	70,935
Securities purchased under resale agreements	3,241	1,956
Investment securities held-to-maturity	23,318	17,439
Net loans (excluding leases)	22,431	18,781
Financial Liabilities:
Non-interest-bearing	47,175	59,397
Interest-bearing—U.S.	50,139	30,911
Interest-bearing—non-U.S.	87,582	96,855
Securities sold under repurchase agreements	2,842	4,400
Other short-term borrowings	1,144	1,585
Long-term debt	11,639	11,282
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	155
Net loans (excluding leases)	51	96
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	280	$ 336
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 3