Fair Value - Carrying Value and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Financial Assets:
Cash and due from banks	$ 3,660	$ 1,855	$ 4,146	$ 3,220
Interest-bearing deposits with banks	68,361	93,523
Securities purchased under resale agreements	9,155	2,390
Investment securities held to maturity, fair value	17,536	17,842
Net loans (excluding leases)	18,081	17,131
Financial Liabilities:
Noninterest-bearing	58,426	70,490
Interest-bearing—U.S.	30,407	33,012
Interest-bearing—non-U.S.	97,534	105,538
Securities sold under repurchase agreements	7,760	8,925
Federal funds purchased	25	21
Other short-term borrowings	3,761	4,381
Long-term debt	11,998	10,229
Reported Amount
Financial Assets:
Cash and due from banks	3,660	1,855
Interest-bearing deposits with banks	68,361	93,523
Securities purchased under resale agreements	9,155	2,390
Investment securities held to maturity, fair value	17,463	17,723
Net loans (excluding leases)	18,108	17,158
Financial Liabilities:
Noninterest-bearing	58,426	70,490
Interest-bearing—U.S.	30,407	33,012
Interest-bearing—non-U.S.	97,534	105,538
Securities sold under repurchase agreements	7,760	8,925
Federal funds purchased	25	21
Other short-term borrowings	3,761	4,381
Long-term debt	12,025	10,042
Quoted Market Prices in Active Markets (Level 1)
Financial Assets:
Cash and due from banks	3,660	1,855
Interest-bearing deposits with banks	0	0
Securities purchased under resale agreements	0	0
Investment securities 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
Pricing Methods with Significant Observable Market Inputs (Level 2)
Financial Assets:
Cash and due from banks	0	0
Interest-bearing deposits with banks	68,361	93,523
Securities purchased under resale agreements	9,155	2,390
Investment securities held to maturity, fair value	17,536	17,842
Net loans (excluding leases)	17,925	16,964
Financial Liabilities:
Noninterest-bearing	58,426	70,490
Interest-bearing—U.S.	30,407	33,012
Interest-bearing—non-U.S.	97,534	105,538
Securities sold under repurchase agreements	7,760	8,925
Federal funds purchased	25	21
Other short-term borrowings	3,761	4,381
Long-term debt	11,448	9,382
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, fair value	0	0
Net loans (excluding leases)	156	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	$ 550	$ 847