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Related Party Transactions Related Party Transactions - Additional Information (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024
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Related Party Transactions [Abstract] |  |
Receivable balance due from a direct wholly-owned subsidiary | $ 9,172 | $ 9,225

X
- Definition Receivable Due From Affiliates + References No definition available. + Details
Name: | fr_ReceivableDueFromAffiliates
Namespace Prefix: | fr_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_RelatedPartyTransactionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration