Cash Equivalents And Marketable Securities - Summary of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	$ 56,779	$ 57,052
Cash equivalents, Fair Value	56,779	57,052
Marketable securities, Amortized Cost	68,780	55,864
Marketable securities, Gross Unrealized Gains	4	2
Marketable securities, Gross Unrealized Losses	(13)	(4)
Marketable securities, Fair Value	68,771	55,862
Total cash equivalents and marketable securities, Amortized Cost	125,559	112,916
Total cash equivalents and marketable securities, Gross Unrealized Gains	4	2
Total cash equivalents and marketable securities, Gross Unrealized Losses	(13)	(4)
Total cash equivalents and marketable securities, Fair Value	125,550	112,914
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	46,879	57,052
Cash equivalents, Fair Value	46,879	57,052
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	8,599
Cash equivalents, Fair Value	8,599
Marketable securities, Amortized Cost	46,032	32,523
Marketable securities, Gross Unrealized Gains	3
Marketable securities, Gross Unrealized Losses	(8)
Marketable securities, Fair Value	46,027	32,523
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Cash equivalents, Amortized Cost	1,301
Cash equivalents, Fair Value	1,301
Marketable securities, Amortized Cost	22,748	23,341
Marketable securities, Gross Unrealized Gains	1	2
Marketable securities, Gross Unrealized Losses	(5)	(4)
Marketable securities, Fair Value	$ 22,744	$ 23,339

Cash and cash equivalents amortized cost.

No definition available.

Name:	fulc_CashAndCashEquivalentsAmortizedCost
Namespace Prefix:	fulc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents and marketable securities accumulated gross unrealized gains before tax.

+ References

No definition available.

+ Details

Name:	fulc_CashEquivalentsAndMarketableSecuritiesAccumulatedGrossUnrealizedGainsBeforeTax
Namespace Prefix:	fulc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Cash equivalents and marketable securities accumulated gross unrealized gains before tax.

+ References

No definition available.

+ Details

Name:	fulc_CashEquivalentsAndMarketableSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	fulc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents and marketable securities amortized cost basis.

+ References

No definition available.

+ Details

Name:	fulc_CashEquivalentsAndMarketableSecuritiesAmortizedCostBasis
Namespace Prefix:	fulc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash equivalents and marketable securities fair value disclosure.

+ References

No definition available.

+ Details

Name:	fulc_CashEquivalentsAndMarketableSecuritiesFairValueDisclosure
Namespace Prefix:	fulc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: