Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Financial Assets:
Cash and due from banks	$ 3,939	$ 1,314	$ 3,490	$ 1,207
Interest-bearing deposits with banks	60,956	70,935
Securities purchased under resale agreements	3,465	1,956
Investment securities held-to-maturity	36,836	34,994
Financial Liabilities:
Non-interest-bearing	49,850	59,397
Interest-bearing—U.S.	49,394	30,911
Interest-bearing—non-U.S.	80,019	96,855
Securities sold under repurchase agreements	3,867	4,400
Other short-term borrowings	1,253	1,585
Reported Amount
Financial Assets:
Cash and due from banks	3,939	1,314
Interest-bearing deposits with banks	60,956	70,935
Securities purchased under resale agreements	3,465	1,956
Investment securities held-to-maturity	36,850	35,169
Net loans (excluding leases)	22,868	18,862
Financial Liabilities:
Non-interest-bearing	49,850	59,397
Interest-bearing—U.S.	49,394	30,911
Interest-bearing—non-U.S.	80,019	96,855
Securities sold under repurchase agreements	3,867	4,400
Other short-term borrowings	1,253	1,585
Long-term debt	11,716	11,430
Estimated Fair Value
Financial Assets:
Cash and due from banks	3,939	1,314
Interest-bearing deposits with banks	60,956	70,935
Securities purchased under resale agreements	3,465	1,956
Investment securities held-to-maturity	36,836	34,994
Net loans (excluding leases)	22,866	18,877
Financial Liabilities:
Non-interest-bearing	49,850	59,397
Interest-bearing—U.S.	49,394	30,911
Interest-bearing—non-U.S.	80,019	96,855
Securities sold under repurchase agreements	3,867	4,400
Other short-term borrowings	1,253	1,585
Long-term debt	12,022	11,618
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,939	1,314
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	17,362	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	60,956	70,935
Securities purchased under resale agreements	3,465	1,956
Investment securities held-to-maturity	19,351	17,439
Net loans (excluding leases)	22,811	18,781
Financial Liabilities:
Non-interest-bearing	49,850	59,397
Interest-bearing—U.S.	49,394	30,911
Interest-bearing—non-U.S.	80,019	96,855
Securities sold under repurchase agreements	3,867	4,400
Other short-term borrowings	1,253	1,585
Long-term debt	11,725	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)	55	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	$ 297	$ 336