Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,345	$ 2,469
Total realized and unrealized gain (losses) recorded in revenue	(3)	41
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	(9)
Purchases	1,776	1,394
Settlements	(605)	(1,370)
Transfers into Level 3	372
Transfers out of Level 3	(409)	(1,032)
Fair value, end of period	2,127	1,345
Change in unrealized gains (losses) related to financial instruments held	(3)	5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(350)	(148)
Derivative instruments, assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	8	5
Total realized and unrealized gain (losses) recorded in revenue	(7)	9
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	4	3
Settlements	(4)	(9)
Transfers into Level 3	0
Transfers out of Level 3	0	0
Fair value, end of period	1	8
Change in unrealized gains (losses) related to financial instruments held	(3)	5
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	8	5
Total realized and unrealized gain (losses) recorded in revenue	(7)	9
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	4	3
Settlements	(4)	(9)
Transfers into Level 3	0
Transfers out of Level 3	0	0
Fair value, end of period	1	8
Change in unrealized gains (losses) related to financial instruments held	(3)	5
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	1,337	2,464
Total realized and unrealized gain (losses) recorded in revenue	4	32
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	(9)
Purchases	1,772	1,391
Settlements	(601)	(1,361)
Transfers into Level 3	372
Transfers out of Level 3	(409)	(1,032)
Fair value, end of period	2,126	1,337
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(350)	(148)
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	0
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	0	325
Settlements	0	0
Transfers into Level 3	25
Transfers out of Level 3	(25)	(325)
Fair value, end of period	0	0
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	97	189
Total realized and unrealized gain (losses) recorded in revenue	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	3
Purchases	200	0
Settlements	0	0
Transfers into Level 3	0
Transfers out of Level 3	(298)	(96)
Fair value, end of period	0	97
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	905	1,764
Total realized and unrealized gain (losses) recorded in revenue	3	31
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	(23)
Purchases	1,035	469
Settlements	(620)	(1,254)
Transfers into Level 3	275
Transfers out of Level 3	0	0
Fair value, end of period	1,358	905
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(240)	(82)
Total asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,002	1,953
Total realized and unrealized gain (losses) recorded in revenue	3	32
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	(20)
Purchases	1,235	469
Settlements	(620)	(1,254)
Transfers into Level 3	275
Transfers out of Level 3	(298)	(96)
Fair value, end of period	1,358	1,002
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(240)	(82)
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
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	0
Purchases	119	90
Settlements	2	0
Transfers into Level 3	0
Transfers out of Level 3	0	(90)
Fair value, end of period	119	0
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	32	174
Total realized and unrealized gain (losses) recorded in revenue	1	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	370	196
Settlements	(11)	(60)
Transfers into Level 3	67
Transfers out of Level 3	(47)	(278)
Fair value, end of period	402	32
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(10)	0
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	248	255
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	1	0
Purchases	5	222
Settlements	31	(7)
Transfers into Level 3	0
Transfers out of Level 3	0	(222)
Fair value, end of period	204	248
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(81)	0
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	280	429
Total realized and unrealized gain (losses) recorded in revenue	1	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	(1)	0
Purchases	494	508
Settlements	22	(67)
Transfers into Level 3	67
Transfers out of Level 3	(47)	(590)
Fair value, end of period	725	280
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(91)	0
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	39	33
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	2	9
Purchases	0	0
Settlements	(3)	(3)
Transfers into Level 3	5
Transfers out of Level 3	0	0
Fair value, end of period	43	39
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	16	39
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	(1)	2
Purchases	24	89
Settlements	0	(27)
Transfers into Level 3	0
Transfers out of Level 3	(39)	(21)
Fair value, end of period	0	16
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	(66)
Other U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	0	10
Total realized and unrealized gain (losses) recorded in revenue	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	19	0
Settlements	0	(10)
Transfers into Level 3	0
Transfers out of Level 3	0	0
Fair value, end of period	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(19)	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	5,970	10,682
Derivative Liability	5,882	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	5,970	10,456
Derivative Liability	5,498	5,500
Recurring | Interest-rate contracts(6)
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	0	226
Derivative Liability	100	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	11,850	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	11,467	15,948
Pricing Methods with Significant Observable Market Inputs (Level 2) | Recurring | Interest-rate contracts(6)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	100	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	1	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	1	8
Pricing Methods with Significant Unobservable Market Inputs (Level 3) | Recurring | Interest-rate contracts(6)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	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	1	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
Quoted Market Prices in Active Markets (Level 1) | Recurring | Interest-rate contracts(6)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability	$ 0	$ 0