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Stockholders' equity and employee benefit plans (Employee Benefit Plans) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Stockholders' Equity and Employee Benefit Plans Disclosure [Abstract] |  |  |
Employee contribution benefit plan expense | $ 1.4 | $ 2.2 | $ 10.8
Allowable purchase interval duration | 6 months |  |
Price per share to fair market value | 85.00% |  |

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- Definition Employee Stock Ownership Plan (ESOP), Allowable Purchase Interval Duration + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Employee Stock Ownership Plan, Share Price to Fair Market Value + References No definition available. + Details
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Namespace Prefix: | fet_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Stockholders' Equity and Employee Benefit Plans Disclosure [Abstract] + References No definition available. + Details
Name: | fet_StockholdersEquityAndEmployeeBenefitPlansDisclosureAbstract
Namespace Prefix: | fet_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration