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Inventory (narrative) (detail) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Mar. 31, 2017 | Mar. 31, 2016 | Mar. 31, 2015
Inventory Disclosure [Abstract] |  |  |
Inventory, LIFO Reserve, Effect on Income, Net | $ 3.9 | $ 16.1 | $ 6.9
Effect Of LIFO Earnings Per Share Basic | $ 0.4 | $ 1.62 | $ 0.64
Effect Of LIFO Earnings Per Share Diluted | $ 0.4 | $ 1.6 | $ 0.63

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- Definition The effect of using the LIFO method on the amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period. + References No definition available. + Details
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- Definition The effect of using the LIFO methond on the amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References No definition available. + Details
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Namespace Prefix: | senea_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The difference between the change in the inventory reserve representing the cumulative difference in cost between the first in, first out and the last in, first out inventory valuation methods and any LIFO decrement which have been reflected in the statement of income during the period. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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