Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Financial Assets:
Cash and due from banks	$ 3,735	$ 1,207	$ 3,149	$ 1,855
Interest-bearing deposits with banks	65,032	75,338
Securities purchased under resale agreements	3,722	3,404
Held to maturity, fair value	31,555	29,798
Financial Liabilities:
Noninterest-bearing	54,248	65,800
Interest-bearing—U.S.	31,159	29,958
Interest-bearing—non-U.S.	100,109	95,869
Securities sold under repurchase agreements	4,224	4,499
Federal funds purchased	23	6
Other short-term borrowings	1,683	1,748
Reported Value Measurement
Financial Assets:
Cash and due from banks	3,735	1,207
Interest-bearing deposits with banks	65,032	75,338
Securities purchased under resale agreements	3,722	3,404
Held to maturity, fair value	31,212	29,952
Net loans (excluding leases)	18,242	17,838
Financial Liabilities:
Noninterest-bearing	54,248	65,800
Interest-bearing—U.S.	31,159	29,958
Interest-bearing—non-U.S.	100,109	95,869
Securities sold under repurchase agreements	4,224	4,499
Federal funds purchased	23	6
Other short-term borrowings	1,683	1,748
Long-term debt	10,323	11,497
Estimate of Fair Value Measurement
Financial Assets:
Cash and due from banks	3,735	1,207
Interest-bearing deposits with banks	65,032	75,338
Securities purchased under resale agreements	3,722	3,404
Held to maturity, fair value	31,555	29,798
Net loans (excluding leases)	18,211	17,792
Financial Liabilities:
Noninterest-bearing	54,248	65,800
Interest-bearing—U.S.	31,159	29,958
Interest-bearing—non-U.S.	100,109	95,869
Securities sold under repurchase agreements	4,224	4,499
Federal funds purchased	23	6
Other short-term borrowings	1,683	1,748
Long-term debt	10,285	11,604
Estimate of Fair Value Measurement | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,735	1,207
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Held to maturity, fair value	0	0
Net loans (excluding leases)	0	0
Financial Liabilities:
Noninterest-bearing	0	0
Interest-bearing—U.S.	0	0
Interest-bearing—non-U.S.	0	0
Securities sold under repurchase agreements	0	0
Federal funds purchased	0	0
Other short-term borrowings	0	0
Long-term debt	0	0
Estimate of Fair Value Measurement | Pricing Methods with Significant Observable Market Inputs (Level 2)
Financial Assets:
Cash and due from banks	0	0
Interest-bearing deposits with banks	65,032	75,338
Securities purchased under resale agreements	3,722	3,404
Held to maturity, fair value	31,555	29,798
Net loans (excluding leases)	18,100	17,667
Financial Liabilities:
Noninterest-bearing	54,248	65,800
Interest-bearing—U.S.	31,159	29,958
Interest-bearing—non-U.S.	100,109	95,869
Securities sold under repurchase agreements	4,224	4,499
Federal funds purchased	23	6
Other short-term borrowings	1,683	1,748
Long-term debt	9,905	11,215
Estimate of Fair Value Measurement | 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
Held to maturity, fair value	0	0
Net loans (excluding leases)	111	125
Financial Liabilities:
Noninterest-bearing	0	0
Interest-bearing—U.S.	0	0
Interest-bearing—non-U.S.	0	0
Securities sold under repurchase agreements	0	0
Federal funds purchased	0	0
Other short-term borrowings	0	0
Long-term debt	$ 380	389
Fair Value, Measurements, Nonrecurring | Estimate of Fair Value Measurement
Financial Assets:
Net loans (excluding leases)		$ 14