Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financial Assets:
Cash and due from banks	$ 1,207	$ 1,855	$ 3,220	$ 2,590
Interest-bearing deposits with banks	75,338	93,523
Securities purchased under resale agreements	3,404	2,390
Held to maturity, fair value	29,798	17,842
Financial Liabilities:
Noninterest-bearing	65,800	70,490
Interest-bearing—U.S.	29,958	33,012
Interest-bearing—non-U.S.	95,869	105,538
Securities sold under repurchase agreements	4,499	8,925
Federal funds purchased	6	21
Other short-term borrowings	1,748	4,381
Reported Value Measurement
Financial Assets:
Cash and due from banks	1,207	1,855
Interest-bearing deposits with banks	75,338	93,523
Securities purchased under resale agreements	3,404	2,390
Held to maturity, fair value	29,952	17,723
Net loans (excluding leases)	17,838	17,158
Financial Liabilities:
Noninterest-bearing	65,800	70,490
Interest-bearing—U.S.	29,958	33,012
Interest-bearing—non-U.S.	95,869	105,538
Securities sold under repurchase agreements	4,499	8,925
Federal funds purchased	6	21
Other short-term borrowings	1,748	4,381
Long-term debt	11,534	10,042
Estimate of Fair Value Measurement
Financial Assets:
Cash and due from banks	1,207	1,855
Interest-bearing deposits with banks	75,338	93,523
Securities purchased under resale agreements	3,404	2,390
Held to maturity, fair value	29,798	17,842
Net loans (excluding leases)	17,792	17,131
Financial Liabilities:
Noninterest-bearing	65,800	70,490
Interest-bearing—U.S.	29,958	33,012
Interest-bearing—non-U.S.	95,869	105,538
Securities sold under repurchase agreements	4,499	8,925
Federal funds purchased	6	21
Other short-term borrowings	1,748	4,381
Long-term debt	11,604	10,229
Estimate of Fair Value Measurement | Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	1,207	1,855
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	75,338	93,523
Securities purchased under resale agreements	3,404	2,390
Held to maturity, fair value	29,798	17,842
Net loans (excluding leases)	17,667	16,964
Financial Liabilities:
Noninterest-bearing	65,800	70,490
Interest-bearing—U.S.	29,958	33,012
Interest-bearing—non-U.S.	95,869	105,538
Securities sold under repurchase agreements	4,499	8,925
Federal funds purchased	6	21
Other short-term borrowings	1,748	4,381
Long-term debt	11,215	9,382
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)	125	167
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	389	$ 847
Fair Value, Measurements, Nonrecurring | Estimate of Fair Value Measurement
Financial Assets:
Net loans (excluding leases)	$ 14