Commitments, Guarantees, and Contingencies (Narrative) (Details) - To Be Announced Security Contracts $ in Millions	Sep. 30, 2020 USD ($)
Loss Contingencies [Line Items]
Fair value of the TBA securities	$ 119.5
Purchase Commitments
Loss Contingencies [Line Items]
Estimated fair value of the purchase commitments	$ 119.5

Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(25))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant