Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,350	$ 2,398	$ 1,345	$ 2,469
Total realized and unrealized gain (losses) recorded in revenue	1	13	(3)	33
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	2	0	(9)
Purchases	409	553	844	1,170
Settlements	0	(569)	(410)	(828)
Transfers into Level 3	215		368
Transfers out of Level 3		(54)	(168)	(492)
Fair value, end of period	1,886	2,343	1,886	2,343
Change in unrealized gains (losses) related to financial instruments held	(2)	0	(1)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(90)		(90)
Derivative instruments, assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	5		8	5
Total realized and unrealized gain (losses) recorded in revenue	0		(6)	3
Total realized and unrealized gain (losses) recorded in other comprehensive income	0		0	0
Purchases	2		4	0
Settlements	(5)		(4)	(8)
Transfers into Level 3	0		0
Transfers out of Level 3			0	0
Fair value, end of period	2	0	2	0
Change in unrealized gains (losses) related to financial instruments held	(2)		(1)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	5		8	5
Total realized and unrealized gain (losses) recorded in revenue	0		(6)	3
Total realized and unrealized gain (losses) recorded in other comprehensive income	0		0	0
Purchases	2		4	0
Settlements	(5)		(4)	(8)
Transfers into Level 3	0		0
Transfers out of Level 3			0	0
Fair value, end of period	2	0	2	0
Change in unrealized gains (losses) related to financial instruments held	(2)		(1)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Investment securities available-for-sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,345	2,398	1,337	2,464
Total realized and unrealized gain (losses) recorded in revenue	1	13	3	30
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	2	0	(9)
Purchases	407	553	840	1,170
Settlements	5	(569)	(406)	(820)
Transfers into Level 3	215		368
Transfers out of Level 3		(54)	(168)	(492)
Fair value, end of period	1,884	2,343	1,884	2,343
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(90)		(90)
US Treasury and federal agencies, mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	0	25	0	0
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	0	25	0	325
Settlements	0	0	0	0
Transfers into Level 3	25		25
Transfers out of Level 3		0	0	(275)
Fair value, end of period	25	50	25	50
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Asset-backed securities, student loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	0	190	97	189
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	2	1	2
Purchases	200	0	200	0
Settlements	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3		0	(98)	0
Fair value, end of period	200	192	200	192
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Asset-backed securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	951	1,710	905	1,764
Total realized and unrealized gain (losses) recorded in revenue	1	13	2	29
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	(8)	0	(21)
Purchases	60	118	415	250
Settlements	(2)	(560)	(412)	(749)
Transfers into Level 3	175		276
Transfers out of Level 3		0	0	0
Fair value, end of period	1,186	1,273	1,186	1,273
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Total asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	951	1,900	1,002	1,953
Total realized and unrealized gain (losses) recorded in revenue	1	13	2	30
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	(6)	1	(19)
Purchases	260	118	615	250
Settlements	(2)	(560)	(412)	(749)
Transfers into Level 3	175		276
Transfers out of Level 3		0	(98)	0
Fair value, end of period	1,386	1,465	1,386	1,465
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Non-U.S. debt securities, mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	0		0	0
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	118	90	118	90
Settlements	0	0	0	0
Transfers into Level 3	0		0
Transfers out of Level 3		0	0	0
Fair value, end of period	118	90	118	90
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Non-U.S. debt securities, asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	63	111	32	174
Total realized and unrealized gain (losses) recorded in revenue	0	0	1	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	(1)	(1)
Purchases	29	90	60	196
Settlements	0	(9)	(21)	(43)
Transfers into Level 3	15		67
Transfers out of Level 3		(54)	(31)	(188)
Fair value, end of period	97	138	97	138
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(10)		(10)
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	274	261	248	255
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	0	194	5	223
Settlements	7	3	28	9
Transfers into Level 3	0		0
Transfers out of Level 3		0	0	(29)
Fair value, end of period	201	458	201	458
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(80)		(80)
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	337	372	280	429
Total realized and unrealized gain (losses) recorded in revenue	0	0	1	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	(1)	(1)
Purchases	147	374	183	509
Settlements	7	(6)	7	(34)
Transfers into Level 3	15		67
Transfers out of Level 3		(54)	(31)	(217)
Fair value, end of period	416	686	416	686
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(90)		(90)
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	38	33	39	33
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	7	0	9
Purchases	0	0	0	0
Settlements	0	0	(1)	(2)
Transfers into Level 3	0		0
Transfers out of Level 3		0	0	0
Fair value, end of period	38	40	38	40
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Collateralized mortgage obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		68	16	39
Total realized and unrealized gain (losses) recorded in revenue		0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income		1	0	2
Purchases		36	23	86
Settlements		(3)	0	(25)
Transfers into Level 3			0
Transfers out of Level 3		0	(39)	0
Fair value, end of period	0	102	0	102
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0
Other U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	19		0	10
Total realized and unrealized gain (losses) recorded in revenue	0		0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0		0	0
Purchases	0		19	0
Settlements	0		0	(10)
Transfers into Level 3	0		0
Transfers out of Level 3			0	0
Fair value, end of period	19	$ 0	19	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	7,466		7,466		$ 10,682
Derivative Liability	4,543		4,543		6,002
Recurring | Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	7,465		7,465		10,456
Derivative Liability	4,118		4,118		5,500
Recurring | Interest-rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	1		1		226
Derivative Liability	106		106		122
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	11,997		11,997		16,676
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	11,581		11,581		15,948
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Interest-rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	97		97		348
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	2		2		8
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	2		2		8
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Interest-rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	0		0		0
Quoted Market Prices in Active Markets (Level 1) | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	10		10		0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	0		0		0
Quoted Market Prices in Active Markets (Level 1) | Recurring | Interest-rate contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	$ 10		$ 10		$ 0