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Commitments, Contingencies and Guarantees (Narrative) (Details) - USD ($)		3 Months Ended
	Aug. 10, 2020	Mar. 31, 2021
Commitment And Contingencies [Line Items]
Aggregate loss on securities		$ 44,000,000
Maximum aggregate loss, if securities' price falls to zero and no debts collected		50,000,000
Guarantees, Fair Value Disclosure		$ 0
FINRA [Member]
Commitment And Contingencies [Line Items]
Settlement, penalties agreed to paid	$ 15,000,000
SEC [Member]
Commitment And Contingencies [Line Items]
Settlement, penalties agreed to paid	11,500,000
CFTC [Member]
Commitment And Contingencies [Line Items]
Settlement, penalties agreed to paid	11,500,000
Disgorgement [Member]
Commitment And Contingencies [Line Items]
Settlement, penalties agreed to paid	$ 700,000

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- Definition

Commitment And Contingencies [Line Items]

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- Definition

Maximum Aggregate Loss, If Securities' Price Falls To Zero And No Debts Collected

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- Definition

Fair value portion of guarantee, including, but not limited to, financial standby letter of credit, fair value guarantee, guarantee of collection of scheduled contractual cash flows from financial assets, and contracts that contingently require the guarantor to make payments or perform services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121555522&loc=d3e12201-110248

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Amount awarded to other party in judgment or settlement of litigation.

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Amount of unrealized and realized gain (loss) on investment in marketable security, including other-than-temporary impairment (OTTI).

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