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Investments (Schedule of Unrealized Appreciation) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Investments [Abstract]    
Net change in unrealized appreciation on available-for-sale securities before deferred tax $ 553us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss $ 3,123us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Deferred income tax (188)nsec_ChangeInDeferredTaxUnrealizedAppreciation (1,062)nsec_ChangeInDeferredTaxUnrealizedAppreciation
Net change in unrealized appreciation on available-for-sale securities $ 365us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax $ 2,061us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax