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Stockholders' Equity (Share-Based Payments - General Descriptions) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jun. 20, 2008
Jun. 25, 2004
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Adjusted Capital $ 1,185,643,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance $ 1,126,232,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance $ 1,004,766,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance    
Pre-tax compensation cost (benefit) recognized 4,400,000us-gaap_AllocatedShareBasedCompensationExpense 5,700,000us-gaap_AllocatedShareBasedCompensationExpense 1,100,000us-gaap_AllocatedShareBasedCompensationExpense    
Expiration period of awards 10 years        
Period in force, from which option holders may elect to sell back acquired shares 90 days        
Compensation cost related to nonvested options not yet recognized, weighted-average period of recognition 2 years 3 months 18 days        
Compensation cost, liability balance 9,300,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized 5,900,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized      
Compensation cost (benefit), tax expense (benefit) (1,500,000)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense (2,000,000)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense (400,000)us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense    
Period of recognition 2,300,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions 3,700,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions 1,700,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions    
Common Class A [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Common stock, par value $ 1us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
$ 1us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
  $ 1.00us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
$ 1.00us-gaap_CommonStockParOrStatedValuePerShare
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Stock and Incentive Plan, 1995 Plan [Member] | Common Class A [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based payments, number of shares authorized under plans         300,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= nwli_StockAndIncentivePlan1995PlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Stock and Incentive Plan, 2008 Plan [Member] | Common Class A [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based payments, number of shares authorized under plans       300,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= nwli_StockAndIncentivePlan2008PlanMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
 
Director [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Requisite service period of awards 1 year        
Annual vesting percentage 20.00%nwli_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualVestingPercentage
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember
       
Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Requisite service period of awards 3 years        
Annual vesting percentage 20.00%nwli_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualVestingPercentage
/ us-gaap_TitleOfIndividualAxis
= nwli_EmployeesMember
       
Colorado Division of Insurance [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Current authorized control level RBC 72,400,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusRequired
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= nwli_ColoradoDivisionOfInsuranceMember
       
Adjusted Capital $ 1,300,000,000us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
/ us-gaap_StatutoryAccountingPracticesByJurisdictionAxis
= nwli_ColoradoDivisionOfInsuranceMember