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Investments (Major Categories of Investment Gains and Losses) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Gain (Loss) on Investments [Line Items]    
Net realized investment gains (losses) $ 100us-gaap_RealizedInvestmentGainsLosses $ 4,439us-gaap_RealizedInvestmentGainsLosses
Impairment of real estate 552us-gaap_ImpairmentOfRealEstate  
Fixed Maturities [Member]    
Gain (Loss) on Investments [Line Items]    
Net realized investment gains (losses) 478us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_FixedMaturitiesMember		 (95)us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_FixedMaturitiesMember
Equity Securities [Member]    
Gain (Loss) on Investments [Line Items]    
Net realized investment gains (losses) 0us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_EquitySecuritiesMember		 1,219us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= us-gaap_EquitySecuritiesMember
Other, Principally Real Estate [Member]    
Gain (Loss) on Investments [Line Items]    
Net realized investment gains (losses) $ (378)us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= nsec_OtherPrincipallyRealEstateMember		 $ 3,315us-gaap_RealizedInvestmentGainsLosses
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= nsec_OtherPrincipallyRealEstateMember