Stock-based Compensation - Stock Units Activity (Q1) (Details) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2024	Dec. 31, 2023
RSUs
Number of RSUs and PSUs
Beginning balance (in shares)	7,683,598	6,269,868
Granted (in shares)	151,516	6,634,013
Vested (in shares)	(629,884)	(2,245,844)
Forfeited (in shares)	(710,060)	(2,974,439)
Ending balance (in shares)	6,495,170	7,683,598
Weighted-average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 2.96	$ 7.07
Granted (in dollars per share)	0.99	1.03
Vested (in dollars per share)	4.03	6.66
Forfeited (in dollars per share)	2.37	4.53
Ending balance (in dollars per share)	$ 2.87	$ 2.96
PRSUs
Number of RSUs and PSUs
Beginning balance (in shares)	234,375	859,375
Granted (in shares)	0	0
Vested (in shares)	0	0
Forfeited (in shares)	0	(625,000)
Ending balance (in shares)	234,375	234,375
Weighted-average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 6.4	$ 6.4
Granted (in dollars per share)	0	0
Vested (in dollars per share)	0	0
Forfeited (in dollars per share)	0	6.4
Ending balance (in dollars per share)	$ 6.4	$ 6.4

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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- Definition

Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

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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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No definition available.

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- Definition

Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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- Definition

The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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