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Short-Term Borrowings (narrative) (detail) (USD $)
Mar. 31, 2015
Mar. 31, 2014
Line of Credit Facility [Line Items]    
Line of Credit Facility, Amount Outstanding $ 233,000,000us-gaap_LineOfCredit  
Debt, Weighted Average Interest Rate 1.92%us-gaap_DebtWeightedAverageInterestRate  
Pledged Assets, Not Separately Reported, Other 542,400,000us-gaap_PledgedAssetsNotSeparatelyReportedOtherFinancialInstruments  
Notes Payable 9,903,000us-gaap_NotesPayableToBankCurrent 12,255,000us-gaap_NotesPayableToBankCurrent
Short-term Debt, Weighted Average Interest Rate 1.67%us-gaap_ShortTermDebtWeightedAverageInterestRate 1.65%us-gaap_ShortTermDebtWeightedAverageInterestRate
Production Period [Member]    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Current Borrowing Capacity 400,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ senea_RevolverBorrowingPeriodsAxis
= senea_ProductionPeriodMember
 
Nonproduction Period [Member]    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Current Borrowing Capacity 300,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ senea_RevolverBorrowingPeriodsAxis
= senea_NonproductionPeriodMember
 
Letter of Credit [Member]    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Current Borrowing Capacity $ 11,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ senea_RevolverBorrowingPeriodsAxis
= us-gaap_LetterOfCreditMember
$ 10,600,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ senea_RevolverBorrowingPeriodsAxis
= us-gaap_LetterOfCreditMember