Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,344	$ 1,316	$ 1,288	$ 2,127
Total realized and unrealized gain (losses) recorded in revenue	(3)	4	(4)	3
Total realized and unrealized gain (losses) recorded in other comprehensive income	5	(2)	1	(4)
Purchases	670	273	814	702
Sales	0	(37)	0	(984)
Settlements and Other	(109)	(53)	(180)	(78)
Transfers into Level 3	216		216	68
Transfers out of Level 3	0		(12)	(333)
Fair value, end of period	2,123	1,501	2,123	1,501
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	4	3	4	1
Total realized and unrealized gain (losses) recorded in revenue	(3)	3	(5)	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	10	4	12	5
Sales	0	0	0	0
Settlements and Other	0	(3)	0	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Fair value, end of period	11	7	11	7
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	4	3	4	1
Total realized and unrealized gain (losses) recorded in revenue	(3)	3	(5)	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0	0	0
Purchases	10	4	12	5
Sales	0	0	0	0
Settlements and Other	0	(3)	0	0
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		0	0
Fair value, end of period	11	7	11	7
Change in unrealized gains (losses) related to financial instruments held	(2)	2	(2)	2
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	123		123
Sales	0		0
Settlements and Other	0		0
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Fair value, end of period	123		123
Asset-backed securities, collateralized loan obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	668	826	593	1,358
Total realized and unrealized gain (losses) recorded in revenue	0	1	1	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	2	(2)	0	(3)
Purchases	455	0	587	318
Sales	0	0	0	(636)
Settlements and Other	(119)	26	(175)	21
Transfers into Level 3	216		216	0
Transfers out of Level 3	0		0	(209)
Fair value, end of period	1,222	851	1,222	851
Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	668	826	593	1,358
Total realized and unrealized gain (losses) recorded in revenue	0	1	1	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	2	(2)	0	(3)
Purchases	455	0	587	318
Sales	0	0	0	(636)
Settlements and Other	(119)	26	(175)	21
Transfers into Level 3	216		216	0
Transfers out of Level 3	0		0	(209)
Fair value, end of period	1,222	851	1,222	851
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 and Other				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	627	272	631	402
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	3	0	1	(1)
Purchases	82	269	92	380
Sales	0	0	0	(311)
Settlements and Other	9	(67)	(3)	(64)
Transfers into Level 3	0		0	68
Transfers out of Level 3	0		0	0
Fair value, end of period	721	474	721	474
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	45	178	58	204
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	0
Sales	0	0	0	0
Settlements and Other	1	(9)	0	(35)
Transfers into Level 3	0		0	0
Transfers out of Level 3	0		(12)	0
Fair value, end of period	46	169	46	169
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	672	450	689	725
Total realized and unrealized gain (losses) recorded in revenue	0	0	0	0
Total realized and unrealized gain (losses) recorded in other comprehensive income	3	0	1	(1)
Purchases	82	269	92	380
Sales	0	0	0	(311)
Settlements and Other	10	(76)	(3)	(99)
Transfers into Level 3	0		0	68
Transfers out of Level 3	0		(12)	(119)
Fair value, end of period	767	643	767	643
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		37		43
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		(1)
Sales		(37)		(37)
Settlements and Other		0		0
Transfers into Level 3				0
Transfers out of Level 3				(5)
Fair value, end of period		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	1,340	1,313	1,284	2,126
Total realized and unrealized gain (losses) recorded in revenue	0	1	1	2
Total realized and unrealized gain (losses) recorded in other comprehensive income	5	(2)	1	(4)
Purchases	660	269	802	697
Sales	0	(37)	0	(984)
Settlements and Other	(109)	(50)	(180)	(78)
Transfers into Level 3	216		216	68
Transfers out of Level 3	0		(12)	(333)
Fair value, end of period	2,112	$ 1,494	2,112	$ 1,494
Collateralized mortgage obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period			2
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 and Other			(2)
Transfers into Level 3			0
Transfers out of Level 3			0
Fair value, end of period	$ 0		$ 0