Commitments and contingencies.

No definition available.

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- Definition

Indicates (true false) whether lessee has option to extend operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-3

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of loss contingency liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483076/450-20-50-1

Reference 2: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483076/450-20-50-10

+ Details

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

- Definition

The amount of loss pertaining to the specified contingency that was charged against earnings in the period, including the effects of revisions in previously reported estimates.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483076/450-20-50-1

+ Details

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Balance Type:	debit
Period Type:	duration

- Details

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- Details

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- Details

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Namespace Prefix:
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- Details

Name:	us-gaap_OtherCommitmentsAxis=fulc_CommitmentForOfficeAndLaboratorySpaceLeaseOfCurrentCorporateHeadquartersMember
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Commitments and Contingencies - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Nov. 30, 2017	Dec. 31, 2023	Dec. 31, 2022
Indemnification Agreement
Commitments And Contingencies [Line Items]
Accrued liabilities for indemnification agreements		$ 0	$ 0
Legal Proceedings
Commitments And Contingencies [Line Items]
Expenses incurred for legal proceedings		$ 0
Cambridge, Massachusetts \| Commitment for Office and Laboratory Space Lease of Current Corporate Headquarters
Commitments And Contingencies [Line Items]
Operating lease option to extend	true