Fair Value - Schedule of Fair Value Measurements, Assets, Using Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	$ 1,288	$ 2,127
Total realized and unrealized gain (losses) recorded in revenue	(2)	(1)
Total realized and unrealized gain (losses) recorded in other comprehensive income	(4)	(1)
Purchases	144	432
Sales	0	(946)
Settlements	(70)	(30)
Transfers into Level 3	0	68
Transfers out of Level 3	(12)	(333)
Fair value, end of period	1,344	1,316
Change in unrealized gains (losses) related to financial instruments held	(1)	(2)
Derivative instruments, assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	4	1
Total realized and unrealized gain (losses) recorded in revenue	(3)	(2)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	3	4
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value, end of period	4	3
Change in unrealized gains (losses) related to financial instruments held	(1)	(2)
Foreign exchange contracts
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	4	1
Total realized and unrealized gain (losses) recorded in revenue	(3)	(2)
Total realized and unrealized gain (losses) recorded in other comprehensive income	0	0
Purchases	3	4
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value, end of period	4	3
Change in unrealized gains (losses) related to financial instruments held	(1)	(2)
Asset-backed securities, collateralized loan obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	593	1,358
Total realized and unrealized gain (losses) recorded in revenue	1	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(1)
Purchases	132	318
Sales	0	(636)
Settlements	(56)	(5)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(209)
Fair value, end of period	668	826
Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	593	1,358
Total realized and unrealized gain (losses) recorded in revenue	1	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	(2)	(1)
Purchases	132	318
Sales	0	(636)
Settlements	(56)	(5)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(209)
Fair value, end of period	668	826
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
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	631	402
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	9	110
Sales	0	(310)
Settlements	(11)	2
Transfers into Level 3	0	68
Transfers out of Level 3	0	0
Fair value, end of period	627	272
Non-U.S. debt securities, other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	58	204
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	(1)	(26)
Transfers into Level 3	0	0
Transfers out of Level 3	(12)	0
Fair value, end of period	45	178
Total non-U.S. debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	689	725
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	9	110
Sales	0	(310)
Settlements	(12)	(24)
Transfers into Level 3	0	68
Transfers out of Level 3	(12)	(119)
Fair value, end of period	672	450
State and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period		43
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		(1)
Transfers into Level 3		0
Transfers out of Level 3		(5)
Fair value, end of period		37
Investment securities available-for-sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning of period	1,284	2,126
Total realized and unrealized gain (losses) recorded in revenue	1	1
Total realized and unrealized gain (losses) recorded in other comprehensive income	(4)	(1)
Purchases	141	428
Sales	0	(946)
Settlements	(70)	(30)
Transfers into Level 3	0	68
Transfers out of Level 3	(12)	(333)
Fair value, end of period	1,340	$ 1,313
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	(2)
Transfers into Level 3	0
Transfers out of Level 3	0
Fair value, end of period	$ 0

Change in Unrealized Gains (Losses) Related to Financial Instruments, Assets, Held at Period End

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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