Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,501	$ 1,350	$ 2,127	$ 1,345
Total realized and unrealized gain (losses) recorded in revenue	(1)	1	0	(3)
Total realized and unrealized gain (losses) recorded in other comprehensive income	(4)	1	(8)	0
Purchases	79	409	778	844
Sales		(90)	(983)	(90)
Settlements	(249)	0	(323)	(410)
Transfers into Level 3	54	215	122	368
Transfers out of Level 3	(64)		(397)	(168)
Fair value, end of period	1,316	1,886	1,316	1,886
Change in unrealized gains (losses) related to financial instruments held	(2)	(2)	(2)	(1)
Derivative instruments, assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	7	5	1	8
Total realized and unrealized gain (losses) recorded in revenue	(2)	0	(3)	(6)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	3	2	4	4
Sales		0	0	0
Settlements	(6)	(5)	0	(4)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0		0	0
Fair value, end of period	2	2	2	2
Change in unrealized gains (losses) related to financial instruments held	(2)	(2)	(2)	(1)
Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	7	5	1	8
Total realized and unrealized gain (losses) recorded in revenue	(2)	0	(3)	(6)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	3	2	4	4
Sales		0	0	0
Settlements	(6)	(5)	0	(4)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0		0	0
Fair value, end of period	2	2	2	2
Change in unrealized gains (losses) related to financial instruments held	(2)	(2)	(2)	(1)
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		0
Sales		0		0
Settlements		0		0
Transfers into Level 3		25		25
Transfers out of Level 3				0
Fair value, end of period		25		25
Asset-backed securities, student loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		0		97
Total realized and unrealized gain (losses) recorded in revenue		0		0
Total realized and unrealized gain (losses) recorded in other comprehensive income		0		1
Purchases		200		200
Sales		0		0
Settlements		0		0
Transfers into Level 3		0		0
Transfers out of Level 3				(98)
Fair value, end of period		200		200
Asset-backed securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	851	951	1,358	905
Total realized and unrealized gain (losses) recorded in revenue	1	1	3	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	1	(5)	0
Purchases	0	60	319	415
Sales		0	(636)	0
Settlements	(239)	(2)	(219)	(412)
Transfers into Level 3	0	175	0	276
Transfers out of Level 3	0		(209)	0
Fair value, end of period	611	1,186	611	1,186
Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	851	951	1,358	1,002
Total realized and unrealized gain (losses) recorded in revenue	1	1	3	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	1	(5)	1
Purchases	0	260	319	615
Sales		0	(636)	0
Settlements	(239)	(2)	(219)	(412)
Transfers into Level 3	0	175	0	276
Transfers out of Level 3	0		(209)	(98)
Fair value, end of period	611	1,386	611	1,386
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	119	0
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		118	0	118
Sales		0	0	0
Settlements		0	0	0
Transfers into Level 3		0	0	0
Transfers out of Level 3			(119)	0
Fair value, end of period	0	118	0	118
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	474	63	402	32
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	0	(3)	(1)
Purchases	76	29	455	60
Sales		(10)	(310)	(10)
Settlements	(3)	0	(67)	(21)
Transfers into Level 3	46	15	114	67
Transfers out of Level 3	0		0	(31)
Fair value, end of period	591	97	591	97
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	169	274	204	248
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	0	0	5
Sales		(80)	0	(80)
Settlements	(1)	7	(36)	28
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(64)		(64)	0
Fair value, end of period	104	201	104	201
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	643	337	725	280
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	0	(3)	(1)
Purchases	76	147	455	183
Sales		(90)	(310)	(90)
Settlements	(4)	7	(103)	7
Transfers into Level 3	46	15	114	67
Transfers out of Level 3	(64)		(183)	(31)
Fair value, end of period	695	416	695	416
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		38	43	39
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	0	0
Sales		0	(37)	0
Settlements		0	(1)	(1)
Transfers into Level 3		0	0	0
Transfers out of Level 3			(5)	0
Fair value, end of period	0	38	0	38
Investment securities available-for-sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,494	1,345	2,126	1,337
Total realized and unrealized gain (losses) recorded in revenue	1	1	3	3
Total realized and unrealized gain (losses) recorded in other comprehensive income	(4)	1	(8)	0
Purchases	76	407	774	840
Sales		(90)	(983)	(90)
Settlements	(243)	5	(323)	(406)
Transfers into Level 3	54	215	122	368
Transfers out of Level 3	(64)		(397)	(168)
Fair value, end of period	1,314	1,884	1,314	1,884
Collateralized mortgage obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	0		0	16
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		0	23
Sales			0	0
Settlements	0		0	0
Transfers into Level 3	8		8	0
Transfers out of Level 3	0		0	(39)
Fair value, end of period	$ 8	0	$ 8	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
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		19
Sales		0		0
Settlements		0		0
Transfers into Level 3		0		0
Transfers out of Level 3				0
Fair value, end of period		$ 19		$ 19