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Significant Accounting Policies (narrative) (detail)
$ in Thousands
12 Months Ended
Mar. 31, 2017
USD ($)
Number
Mar. 31, 2016
USD ($)
Mar. 31, 2015
USD ($)
Sales Revenue [Line Items]      
Net Sales $ 1,245,681 $ 1,275,360 $ 1,286,350
Advertising Expense 2,100 2,000 $ 1,700
Deferred Financing Cost, Current 900    
Deferred Finance Costs, Noncurrent $ 200    
Seneca Foods Plants [Member]      
Real Estate Properties [Line Items]      
Number of Real Estate Properties | Number 27    
Seneca Foods Warehouses [Member]      
Real Estate Properties [Line Items]      
Number of Real Estate Properties | Number 31    
General Mills Operations LLC [Member]      
Sales Revenue [Line Items]      
UnshippedProduct $ 43,300    
General Mills Operations LLC [Member] | Sales Revenue Net [Member]      
Sales Revenue [Line Items]      
Net Sales $ 95,800 $ 126,100  
Net Sales Percentage 6.00% 11.00% 13.00%
Top Ten Customers [Member] | Sales Revenue Net [Member]      
Sales Revenue [Line Items]      
Net Sales Percentage 42.00% 48.00% 49.00%
Shipping and Handling Cost [Member] | Sales Revenue Net [Member]      
Sales Revenue [Line Items]      
Net Sales $ 38,000 $ 38,300 $ 38,800