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Inventories (Inventory reserve) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Inventory Valuation Reserves Increase (Decrease) [Roll Forward] |  |  |
Balance at beginning of period | $ 77,888 | $ 29,456 | $ 26,419
Charged to expense | 25,537 | 51,917 | 8,171
Deductions or other | (35,073) | (3,485) | (5,134)
Balance at end of period | $ 68,352 | $ 77,888 | $ 29,456

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- Definition Inventory Valuation Reserves, Deduction and Other Reserve Decrease + References No definition available. + Details
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- Definition Inventory Valuation Reserves Increase (Decrease) [Roll Forward] + References No definition available. + Details
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- Definition Inventory Valuation Reserves, Provision + References No definition available. + Details
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Namespace Prefix: | fet_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of valuation reserve for inventory. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.BB) -URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=68049868&loc=d3e3927-108312 + Details
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