Business Combination Contingent Consideration Arrangements, Change In Amount Of Contingent Consideration Liability, Net

No definition available.

Name:	form_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiabilityNet
Namespace Prefix:	form_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contingent Consideration, Calculation Amount, Multiple of EBIT

+ References

No definition available.

+ Details

Name:	form_ContingentConsiderationCalculationAmountMultipleOfEBIT
Namespace Prefix:	form_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) in the value of a contingent consideration liability, including, but not limited to, differences arising upon settlement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e7008-128479

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsChangeInAmountOfContingentConsiderationLiability1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=form_FRTMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value and Derivative Instruments - Additional Information (Details) $ in Thousands, € in Millions		3 Months Ended		9 Months Ended
	Oct. 09, 2019 EUR (€)	Sep. 26, 2020 USD ($)	Jun. 27, 2020 USD ($)	Sep. 26, 2020 USD ($)	Sep. 28, 2019 USD ($)	Dec. 28, 2019 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Change in amount of contingent consideration		$ (3,600)	$ 1,200	$ (3,771)	$ 0
Contingent consideration, liability		$ 2,966		2,966		$ 5,364
Business Combination Contingent Consideration Arrangements, Change In Amount Of Contingent Consideration Liability, Net				$ (2,400)
FRT
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent Consideration, Calculation Amount, Multiple of EBIT	150.00%
FRT \| Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Contingent consideration, liability \| €	€ 10.3