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Investments (Schedule of Unrealized Appreciation) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Investments [Abstract]    
Net change in unrealized appreciation on available-for-sale securities before deferred tax $ 3,123us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss $ (4,243)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Deferred income tax (1,062)nsec_ChangeInDeferredTaxUnrealizedAppreciation 1,442nsec_ChangeInDeferredTaxUnrealizedAppreciation
Net change in unrealized appreciation on available-for-sale securities $ 2,061us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax $ (2,801)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax