Securities - Securities Pledged and Received as Collateral (Narrative) (Details) - USD ($) $ in Billions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Available for sale and trading securities pledged as collateral	$ 12.7	$ 12.3
Available for sale and trading securities permitted to be sold or repledged	1.6	1.2
Securities received as collateral	29.3	31.8
Securities received as collateral permitted to be sold or repledged	29.3	$ 31.8
Securities received as collateral that have been sold or repledged	$ 28.3

Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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Period Type:	instant

- Definition

Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions where counterparty has the right to re-pledge such.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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- References

No definition available.

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- Definition

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

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- Definition

The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 05
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=64836671&loc=d3e93645-111703

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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