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Deposits with Regulatory Authorities Deposit with Regulatory Authorities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Schedule of Deposit with Regulatory Authorities [Line Items]    
Assets on deposit with state and other regulatory authorities $ 16,568us-gaap_AssetsHeldByInsuranceRegulators $ 16,701us-gaap_AssetsHeldByInsuranceRegulators
Debt securities held to maturity [Member]    
Schedule of Deposit with Regulatory Authorities [Line Items]    
Assets on deposit with state and other regulatory authorities 15,279us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= us-gaap_HeldtomaturitySecuritiesMember		 15,407us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= us-gaap_HeldtomaturitySecuritiesMember
Debt securities available for sale [Member]    
Schedule of Deposit with Regulatory Authorities [Line Items]    
Assets on deposit with state and other regulatory authorities 814us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= nwli_DebtSecuritiesAvailableForSaleMember		 819us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= nwli_DebtSecuritiesAvailableForSaleMember
Short-term investments [Member]    
Schedule of Deposit with Regulatory Authorities [Line Items]    
Assets on deposit with state and other regulatory authorities $ 475us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 $ 475us-gaap_AssetsHeldByInsuranceRegulators
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember