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Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Income Tax Disclosure [Abstract]    
Federal income tax rate applied to pre-tax income/loss 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 34.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Dividends received deduction and tax-exempt interest (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsDividends (2.40%)us-gaap_EffectiveIncomeTaxRateReconciliationDeductionsDividends
Company owned life insurance (0.80%)nsec_EffectiveIncomeTaxRateReconciliationColi (0.80%)nsec_EffectiveIncomeTaxRateReconciliationColi
Small life deduction (3.20%)nsec_EfectiveInomeTaxRateReconciliationSmallLifeDeduction (11.40%)nsec_EfectiveInomeTaxRateReconciliationSmallLifeDeduction
Effective Income Tax Rate Reconciliation AMT 0.00%nsec_EffectiveIncomeTaxRateReconciliationAMT 1.90%nsec_EffectiveIncomeTaxRateReconciliationAMT
Other, net (1.80%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments 0.80%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective federal income tax rate 26.70%us-gaap_EffectiveIncomeTaxRateContinuingOperations 22.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations