FAIR VALUE MEASUREMENTS, Level 3 Changes in Fair Value of Warrant Liabilities (Q3) (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2021
Transfers to/from Fair Value Hierarchy Levels [Abstract]
Transfers into Level 3	$ 0			$ 0
Transfers out of Level 3	0			0
Private Placement Warrants [Member]
Changes in Fair Value of Warrant Liabilities [Roll Forward]
Fair value, beginning of period	5,430,000	$ 4,619,000	$ 5,673,000	5,673,000
Change in fair value	(1,690,000)	811,000	(1,054,000)
Fair value, end of period	3,740,000	$ 5,430,000	$ 4,619,000	3,740,000
Public Warrants [Member]
Changes in Fair Value of Warrant Liabilities [Roll Forward]
Fair value, end of period	$ 5,900,000			$ 5,900,000

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

No definition available.

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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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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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

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as a liability 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
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- References

No definition available.

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