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Stockholders' equity and employee benefit plans (Employee Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	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%

X

- Definition

Employee Stock Ownership Plan (ESOP), Allowable Purchase Interval Duration

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

X

- 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

X

- 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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X

- 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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