Fair Value Measurements - Change in Fair Values of Level 3 Financial Instruments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other investments
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	$ 10	$ 8	$ 12	$ 7
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	2	(1)	0	(1)
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	1
Transfers out of Level 3	0	0	0	0
End of Period Balance	12	7	12	7
Unrealized Gains (Losses) Included in Earnings	2	(1)	0	(1)
Other investments \| Principal investments, Direct
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	1	1	1	1
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	(1)	0	0
Sales	0	1	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
End of Period Balance	1	1	1	1
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Securities available for sale \| Other securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	9	25	11	20
Gains (Losses) Included in Other Comprehensive Income	3	10	1	15
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	0		0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0		0	(24)
End of Period Balance	12	11	12	11
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Interest rate
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	96	3	22	5
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	3	2	22	3
Purchases	0	0	11	0
Sales	0	0	(1)	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	7	0	62	2
Transfers out of Level 3	(54)	(1)	(64)	(6)
End of Period Balance	52	4	52	4
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Credit
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	(23)	(1)	(8)	0
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	6	0	(10)	0
Purchases	0	0	1	(1)
Sales		0	0	0
Settlements	0	0	0	0
Transfers Other	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3		0	0	0
End of Period Balance	(17)	(1)	(17)	(1)
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Other
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	23	4	5	3
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers Other	23	2	41	3
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
End of Period Balance	46	6	46	6
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Trading account assets \| Other mortgage-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	0		0
Gains (Losses) Included in Other Comprehensive Income	0		0
Gains (Losses) Included in Earnings	0		0
Purchases	0		0
Sales	0		0
Settlements	0		0
Transfers Other	0		0
Transfers into Level 3	36		36
Transfers out of Level 3	0		0
End of Period Balance	36		36
Unrealized Gains (Losses) Included in Earnings	0		0
Loans held for sale (residential)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	10	1	0	0
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	(10)	(1)	(10)	(1)
Settlements	0	0	0	0
Transfers Other	0	0	10	1
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
End of Period Balance	0	0	0	0
Unrealized Gains (Losses) Included in Earnings	0	0	0	0
Loans, net of unearned income (residential)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning of Period Balance	3	3	4	3
Gains (Losses) Included in Other Comprehensive Income	0	0	0	0
Gains (Losses) Included in Earnings	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(1)	0
Settlements	0	0	0	0
Transfers Other	2	0	2	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
End of Period Balance	5	3	5	3
Unrealized Gains (Losses) Included in Earnings	$ 0	$ 0	$ 0	$ 0

Fair value measurement with unobservable inputs reconciliation recurring basis asset and liabilities value.

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- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Other Transfers

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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-Section 50
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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of purchases, (sales), issuances and (settlements) 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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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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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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