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Inventories - Inventory reserve (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
Inventory Valuation Reserves Increase (Decrease) [Roll Forward] |  |
Balance at beginning of period | $ 39,291 | $ 62,885
Charged to expense | 2,784 | 2,698
Deductions or other | (3,888) | (26,292)
Balance at end of period | $ 38,187 | $ 39,291

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- Definition Inventory Valuation Reserves, Deduction and Other Reserve Decrease + References No definition available. + Details
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Namespace Prefix: | fet_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Inventory Valuation Reserves Increase (Decrease) [Roll Forward] + References No definition available. + Details
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Balance Type: | na
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- Definition Amount of valuation reserve for inventory. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.BB) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480581/330-10-S99-2 + Details
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- Definition Amount of loss from reductions in inventory due to subsequent measurement adjustments, including, but not limited to, physical deterioration, obsolescence, or changes in price levels. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 330 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483080/330-10-50-2 + Details
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