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.

No definition available.

+ Details

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e9038-108599

+ Details

Name:	us-gaap_NoncurrentAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_Revenues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_GeographicConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_NonUsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_SalesRevenueNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorCustomersAxis=fit_CustomerCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorCustomersAxis=fit_CustomerAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorCustomersAxis=fit_CustomerBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=fit_AmericasExcludingUnitedStatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_EMEAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_AsiaPacificMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_AccountsReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorCustomersAxis=fit_CustomerDMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Customer Information and Other Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 01, 2017	Jul. 02, 2016	Jul. 01, 2017	Jul. 02, 2016	Dec. 31, 2016
Concentration Risk [Line Items]
Revenue	$ 353,299	$ 586,528	$ 652,241	$ 1,091,884
Geographic Concentration Risk [Member] \| Non-US [Member]
Concentration Risk [Line Items]
Long-lived assets including property and equipment	$ 25,000		$ 25,000		$ 30,100
Revenue [Member] \| Customer Concentration Risk [Member] \| Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	14.00%	12.00%	13.00%
Revenue [Member] \| Customer Concentration Risk [Member] \| Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	11.00%	16.00%	11.00%	17.00%
Revenue [Member] \| Customer Concentration Risk [Member] \| Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage		14.00%		12.00%
Revenue [Member] \| Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Revenue	$ 353,299	$ 586,528	$ 652,241	$ 1,091,884
Revenue [Member] \| Geographic Concentration Risk [Member] \| United States [Member]
Concentration Risk [Line Items]
Revenue	199,201	445,192	369,621	796,877
Revenue [Member] \| Geographic Concentration Risk [Member] \| Americas excluding United States [Member]
Concentration Risk [Line Items]
Revenue	24,412	27,375	44,380	50,769
Revenue [Member] \| Geographic Concentration Risk [Member] \| Europe, Middle East, and Africa [Member]
Concentration Risk [Line Items]
Revenue	108,601	99,471	196,373	174,195
Revenue [Member] \| Geographic Concentration Risk [Member] \| APAC [Member]
Concentration Risk [Line Items]
Revenue	$ 21,085	$ 14,490	$ 41,867	$ 70,043
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			19.00%		19.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			19.00%		16.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Customer D [Member]
Concentration Risk [Line Items]
Concentration risk, percentage					12.00%
Accounts Receivable [Member] \| Customer Concentration Risk [Member] \| Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			12.00%