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Note 3 - Fair Values of Assets and Liabilities (Details Textual) (Equity Investments and Promissory Notes Receivable [Member], USD $)
In Millions, unless otherwise specified
May 02, 2015
Equity Investments and Promissory Notes Receivable [Member]
  
Investment Owned, at Cost $ 3.3us-gaap_InvestmentOwnedAtCost
/ us-gaap_InvestmentTypeAxis
= sigm_EquityInvestmentsAndPromissoryNotesReceivableMember