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Summary of Significant Accounting Policies - Segment Reporting and Concentration Risk (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
May 03, 2015
Apr. 27, 2014
Apr. 28, 2013
Accounting Policies [Abstract]      
Number of reportable segments 1us-gaap_NumberOfReportableSegments    
Concentration Risk [Line Items]      
Net assets located overseas 470.8us-gaap_ConcentrationRiskNetAssetsAmountGeographicArea    
Customer Concentration Risk | Accounts Receivable | 10 Largest Customers      
Concentration Risk [Line Items]      
Concentration risk (percentage) 46.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_AccountsReceivableMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_TenLargestCustomersMember		 55.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_AccountsReceivableMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_TenLargestCustomersMember		  
Customer Concentration Risk | Accounts Receivable | Customer A      
Concentration Risk [Line Items]      
Concentration risk (percentage)   12.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_AccountsReceivableMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_CustomerMember		  
Customer Concentration Risk | Accounts Receivable | Customer B      
Concentration Risk [Line Items]      
Concentration risk (percentage)   11.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_AccountsReceivableMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_CustomerBMember		  
Customer Concentration Risk | Revenues | 10 Largest Customers      
Concentration Risk [Line Items]      
Concentration risk (percentage) 55.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_TenLargestCustomersMember		 58.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_TenLargestCustomersMember		 54.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_TenLargestCustomersMember
Customer Concentration Risk | Revenues | Cisco Systems      
Concentration Risk [Line Items]      
Concentration risk (percentage) 14.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_CiscoSystemsMember		 17.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_CiscoSystemsMember		 17.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= fnsr_CiscoSystemsMember