Income Tax (Deferred Income Tax Assets and Liabilities) (detail) (USD $)	Mar. 31, 2015	Mar. 31, 2014
Components of Deferred Tax Assets and Liabilities [Abstract]
Future tax credits	$ 4,021,000	$ 3,042,000
Inventory valuation	2,348,000us-gaap_DeferredTaxAssetsInventory	3,353,000us-gaap_DeferredTaxAssetsInventory
Employee benefits	3,009,000	2,884,000
Insurance	816,000	1,244,000
Otrher comprehensive loss	20,335,000	7,194,000
Interest	46,000senea_Deferredtaxassetsinterest	138,000senea_Deferredtaxassetsinterest
Deferred gain on sale/leaseback	0	26,000
Defined benefit plan	1,372,000senea_Deferredtaxassetspension
Prepaid revenue	701,000us-gaap_DeferredTaxAssetsDeferredIncome	1,118,000us-gaap_DeferredTaxAssetsDeferredIncome
Other deferred	1,364,000us-gaap_DeferredTaxAssetsOther	859,000us-gaap_DeferredTaxAssetsOther
Severance	256,000	87,000
Total	34,268,000us-gaap_DeferredTaxAssetsGross	19,945,000us-gaap_DeferredTaxAssetsGross
Property basis and depreciation difference	9,129,000	10,757,000
481 (a) adjustment	1,281,000
Income from Equity Investment	245,000
Pension		725,000us-gaap_DeferredTaxLiabilitiesOther
Total	10,655,000us-gaap_DeferredTaxLiabilities	11,482,000us-gaap_DeferredTaxLiabilities
Valuation allowance - non-current	1,787,000	390,000
Net deferred income tax asset	$ 21,826,000us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 8,073,000us-gaap_DeferredTaxAssetsLiabilitiesNet

The tax effect as of the balance sheet date of the amount of estimated future tax deductions related to interest.

No definition available.

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The tax effect as of the balance sheet date of the amount of estimated future tax deductions related to the defined benefit plan.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from a gain reported for tax purposes on sale and leaseback transactions in accordance with enacted tax laws.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from deferred income.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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-Glossary Deferred Tax Asset

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from inventory.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from unrealized losses in other comprehensive income.

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The tax effect as of the balance sheet date of the amount of future tax deductions arising from all unused tax credit carryforwards which have been reduced by a valuation allowance.

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-Glossary Carryforwards

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The tax effect as of the balance sheet date of the amount of the estimated future tax deductions arising from employee benefit costs not otherwise specified in the taxonomy, which can only be deducted for tax purposes when actual costs are incurred, and which can only be realized if sufficient tax-basis income is generated in future periods to enable the deduction to be taken.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from other compensation and benefits not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from severance costs.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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Amount of deferred tax liability attributable to taxable temporary differences not separately disclosed.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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Amount of deferred tax liability attributable to taxable temporary differences from regulatory assets.

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Amount of deferred tax liability attributable to taxable temporary differences from tax deferred revenue or other income not separately disclosed.

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