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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Schedule of Deferred Acquisition Costs [Line Items]        
Derivatives, index options $ 114,287,000us-gaap_DerivativeAssets $ 169,314,000us-gaap_DerivativeAssets    
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]        
Deferred policy acquisition costs, beginning of year 785,706,000us-gaap_DeferredPolicyAcquisitionCosts 705,397,000us-gaap_DeferredPolicyAcquisitionCosts 722,542,000us-gaap_DeferredPolicyAcquisitionCosts  
Policy acquisition costs deferred 141,868,000us-gaap_DeferredPolicyAcquisitionCostsAdditions 133,982,000us-gaap_DeferredPolicyAcquisitionCostsAdditions 128,517,000us-gaap_DeferredPolicyAcquisitionCostsAdditions  
Amortization of deferred policy acquisition costs (115,154,000)us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense (108,233,000)us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense (121,922,000)us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense  
Adjustments for unrealized (gains) losses on investment securities (9,501,000)us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseUnrealizedInvestmentGainsLosses 54,560,000us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseUnrealizedInvestmentGainsLosses (23,740,000)us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseUnrealizedInvestmentGainsLosses  
Deferred policy acquisition costs, end of year 802,919,000us-gaap_DeferredPolicyAcquisitionCosts 785,706,000us-gaap_DeferredPolicyAcquisitionCosts 705,397,000us-gaap_DeferredPolicyAcquisitionCosts 722,542,000us-gaap_DeferredPolicyAcquisitionCosts
Movement in Deferred Sales Inducements [Roll Forward]        
Deferred sales inducements, beginning of year 169,570,000us-gaap_DeferredSalesInducementsNet 152,844,000us-gaap_DeferredSalesInducementsNet 155,753,000us-gaap_DeferredSalesInducementsNet  
Sales inducement costs deferred 18,355,000us-gaap_DeferredSalesInducementsAdditions 26,159,000us-gaap_DeferredSalesInducementsAdditions 31,758,000us-gaap_DeferredSalesInducementsAdditions  
Amortization of sales inducements (26,357,000)us-gaap_DeferredSalesInducementsAmortizationExpense (25,357,000)us-gaap_DeferredSalesInducementsAmortizationExpense (27,261,000)us-gaap_DeferredSalesInducementsAmortizationExpense  
Adjustments for unrealized (gains) losses on investment securities (1,802,000)us-gaap_DeferredSalesInducementsAdjustments 15,924,000us-gaap_DeferredSalesInducementsAdjustments (7,406,000)us-gaap_DeferredSalesInducementsAdjustments  
Deferred sales inducements, end of year 159,766,000us-gaap_DeferredSalesInducementsNet 169,570,000us-gaap_DeferredSalesInducementsNet 152,844,000us-gaap_DeferredSalesInducementsNet 155,753,000us-gaap_DeferredSalesInducementsNet
Increase in amortization during the year       1,400,000nwli_DeferredPolicyAcquisitionCostAmortizationExpenseIncreaseDuringYear
True-up adjustments recorded 7,200,000us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseEffectOfAdjustmentsToEstimatedGrossProfit 15,800,000us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseEffectOfAdjustmentsToEstimatedGrossProfit 12,800,000us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseEffectOfAdjustmentsToEstimatedGrossProfit  
True-up adjustment in deferred sales inducement 1,600,000nwli_DeferredSalesInducementIncreaseDecreaseinAmortizationExpenseDuetoTrueupAdjustment 800,000nwli_DeferredSalesInducementIncreaseDecreaseinAmortizationExpenseDuetoTrueupAdjustment 5,500,000nwli_DeferredSalesInducementIncreaseDecreaseinAmortizationExpenseDuetoTrueupAdjustment  
Summary of Premiums and Deposits Collected        
Annuity deposits 833,273,000nwli_AnnuityDepositsCollected 813,868,000nwli_AnnuityDepositsCollected 864,719,000nwli_AnnuityDepositsCollected  
Universal life insurance deposits 280,980,000nwli_UniversalLifeInsuranceDepositsCollected 296,286,000nwli_UniversalLifeInsuranceDepositsCollected 248,118,000nwli_UniversalLifeInsuranceDepositsCollected  
Traditional life and other premiums 22,651,000nwli_TraditionalLifeAndOtherPremiumsCollected 22,164,000nwli_TraditionalLifeAndOtherPremiumsCollected 21,476,000nwli_TraditionalLifeAndOtherPremiumsCollected  
Totals 1,136,904,000nwli_DirectPremiumsAndDepositsCollected 1,132,318,000nwli_DirectPremiumsAndDepositsCollected 1,134,313,000nwli_DirectPremiumsAndDepositsCollected  
Statutory Accounting Practices        
Net gain from operations before Federal and foreign income taxes 95,892,000nwli_StatutoryAccountingPracticesStatutoryNetGainFromOperationsBeforeFederalAndForeignIncomeTaxes 170,796,000nwli_StatutoryAccountingPracticesStatutoryNetGainFromOperationsBeforeFederalAndForeignIncomeTaxes 130,644,000nwli_StatutoryAccountingPracticesStatutoryNetGainFromOperationsBeforeFederalAndForeignIncomeTaxes  
Net income 77,220,000us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount 106,159,000us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount 84,473,000us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount  
Unassigned surplus 1,141,579,000us-gaap_StatutoryAccountingPracticesPortionOfExcessRetainedEarningsNotTaxed 1,082,519,000us-gaap_StatutoryAccountingPracticesPortionOfExcessRetainedEarningsNotTaxed 961,054,000us-gaap_StatutoryAccountingPracticesPortionOfExcessRetainedEarningsNotTaxed  
Capital and surplus 1,185,643,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance 1,126,232,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance 1,004,766,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance  
Agents' Commissions [Member]        
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]        
Policy acquisition costs deferred 134,262,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_AgentsCommissionsMember		 127,161,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_AgentsCommissionsMember		 121,900,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_AgentsCommissionsMember		  
Other Costs [Member]        
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]        
Policy acquisition costs deferred $ 7,606,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_OtherCostsMember		 $ 6,821,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_OtherCostsMember		 $ 6,617,000us-gaap_DeferredPolicyAcquisitionCostsAdditions
/ nwli_DeferredPolicyAcquisitionCostsTypeAxis
= nwli_OtherCostsMember		  
Minimum [Member]        
Movement in Deferred Sales Inducements [Roll Forward]        
Interest rate range to calculate future mortality 3.25%us-gaap_LiabilityForFuturePolicyBenefitByProductSegmentInterestRateYearOfIssue
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Maximum [Member]        
Movement in Deferred Sales Inducements [Roll Forward]        
Interest rate range to calculate future mortality 8.00%us-gaap_LiabilityForFuturePolicyBenefitByProductSegmentInterestRateYearOfIssue
/ us-gaap_RangeAxis
= us-gaap_MaximumMember