Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,316	$ 1,265	$ 2,127	$ 1,345
Total realized and unrealized gain (losses) recorded in revenue	4	3	3	(3)
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(2)	(4)	0
Purchases	273	220	702	439
Sales	(37)		(984)
Settlements	(53)	(119)	(78)	(414)
Transfers into Level 3		152	68	152
Transfers out of Level 3		(169)	(333)	(169)
Fair value, end of period	1,501	1,350	1,501	1,350
Change in unrealized gains (losses) related to financial instruments held	2	2	2	2
Derivative instruments, assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	3	2	1	8
Total realized and unrealized gain (losses) recorded in revenue	3	1	1	(6)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	4	2	5	7
Sales	0		0
Settlements	(3)	0	0	(4)
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fair value, end of period	7	5	7	5
Change in unrealized gains (losses) related to financial instruments held	2	2	2	2
Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	3	2	1	8
Total realized and unrealized gain (losses) recorded in revenue	3	1	1	(6)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	4	2	5	7
Sales	0		0
Settlements	(3)	0	0	(4)
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fair value, end of period	7	5	7	5
Change in unrealized gains (losses) related to financial instruments held	2	2	2	2
Asset-backed securities, student loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		99		97
Total realized and unrealized gain (losses) recorded in revenue		0		0
Total realized and unrealized gain (losses) recorded in other comprehensive income		0		2
Purchases		0		0
Settlements		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		(99)		(99)
Fair value, end of period		0		0
Asset-backed securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	826	771	1,358	905
Total realized and unrealized gain (losses) recorded in revenue	1	1	2	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(1)	(3)	(1)
Purchases	0	199	318	354
Sales	0		(636)
Settlements	26	(120)	21	(410)
Transfers into Level 3		101	0	101
Transfers out of Level 3		0	(209)	0
Fair value, end of period	851	951	851	951
Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	826	870	1,358	1,002
Total realized and unrealized gain (losses) recorded in revenue	1	1	2	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(1)	(3)	1
Purchases	0	199	318	354
Sales	0		(636)
Settlements	26	(120)	21	(410)
Transfers into Level 3		101	0	101
Transfers out of Level 3		(99)	(209)	(99)
Fair value, end of period	851	951	851	951
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			119
Total realized and unrealized gain (losses) recorded in revenue			0
Total realized and unrealized gain (losses) recorded in other comprehensive income			0
Purchases			0
Sales			0
Settlements			0
Transfers into Level 3			0
Transfers out of Level 3			(119)
Fair value, end of period	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	272	59	402	32
Total realized and unrealized gain (losses) recorded in revenue	0	1	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	(1)	(1)	(1)
Purchases	269	0	380	31
Sales	0		(311)
Settlements	(67)	(16)	(64)	(20)
Transfers into Level 3		51	68	51
Transfers out of Level 3		(31)	0	(31)
Fair value, end of period	474	63	474	63
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	178	256	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	0		0
Settlements	(9)	18	(35)	21
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fair value, end of period	169	274	169	274
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	450	315	725	280
Total realized and unrealized gain (losses) recorded in revenue	0	1	0	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	(1)	(1)	(1)
Purchases	269	0	380	36
Sales	0		(311)
Settlements	(76)	2	(99)	1
Transfers into Level 3		51	68	51
Transfers out of Level 3		(31)	(119)	(31)
Fair value, end of period	643	337	643	337
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	37	39	43	39
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	(1)	0
Sales	(37)		(37)
Settlements	0	(1)	0	(1)
Transfers into Level 3		0	0	0
Transfers out of Level 3		0	(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	1,313	1,263	2,126	1,337
Total realized and unrealized gain (losses) recorded in revenue	1	2	2	3
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(2)	(4)	0
Purchases	269	218	697	432
Sales	(37)		(984)
Settlements	(50)	(119)	(78)	(410)
Transfers into Level 3		152	68	152
Transfers out of Level 3		(169)	(333)	(169)
Fair value, end of period	$ 1,494	1,345	$ 1,494	1,345
Collateralized mortgage obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		39		16
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		23
Settlements		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		(39)		(39)
Fair value, end of period		0		0
Other U.S. debt 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		19		19
Settlements		0		0
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Fair value, end of period		$ 19		$ 19