Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Financial Assets:
Cash and due from banks	$ 2,909	$ 1,314	$ 3,735	$ 1,207
Interest-bearing deposits with banks	66,789	70,935
Securities purchased under resale agreements	2,181	1,956
Investment securities held-to-maturity	35,694	34,994
Financial Liabilities:
Non-interest-bearing	56,786	59,397
Interest-bearing—U.S.	26,746	30,911
Interest-bearing—non-U.S.	99,933	96,855
Securities sold under repurchase agreements	4,003	4,400
Other short-term borrowings	1,177	1,585
Reported Amount
Financial Assets:
Cash and due from banks	2,909	1,314
Interest-bearing deposits with banks	66,789	70,935
Securities purchased under resale agreements	2,181	1,956
Investment securities held-to-maturity	35,829	35,169
Net loans (excluding leases)	21,653	18,862
Financial Liabilities:
Non-interest-bearing	56,786	59,397
Interest-bearing—U.S.	26,746	30,911
Interest-bearing—non-U.S.	99,933	96,855
Securities sold under repurchase agreements	4,003	4,400
Other short-term borrowings	1,177	1,585
Long-term debt	11,394	11,430
Estimated Fair Value
Financial Assets:
Cash and due from banks	2,909	1,314
Interest-bearing deposits with banks	66,789	70,935
Securities purchased under resale agreements	2,181	1,956
Investment securities held-to-maturity	35,694	34,994
Net loans (excluding leases)	21,657	18,877
Financial Liabilities:
Non-interest-bearing	56,786	59,397
Interest-bearing—U.S.	26,746	30,911
Interest-bearing—non-U.S.	99,933	96,855
Securities sold under repurchase agreements	4,003	4,400
Other short-term borrowings	1,177	1,585
Long-term debt	11,624	11,618
Estimated Fair Value | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	2,909	1,314
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities held-to-maturity	17,395	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	66,789	70,935
Securities purchased under resale agreements	2,181	1,956
Investment securities held-to-maturity	18,164	17,439
Net loans (excluding leases)	21,567	18,781
Financial Liabilities:
Non-interest-bearing	56,786	59,397
Interest-bearing—U.S.	26,746	30,911
Interest-bearing—non-U.S.	99,933	96,855
Securities sold under repurchase agreements	4,003	4,400
Other short-term borrowings	1,177	1,585
Long-term debt	11,301	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	135	155
Net loans (excluding leases)	90	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	323	$ 336
Nonrecurring Measurement Basis | Estimated Fair Value
Financial Assets:
Net loans (excluding leases)	$ 28