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Derivative Investments (Balance Sheet) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Derivatives, Fair Value [Line Items]    
Derivatives, index options $ 114,287us-gaap_DerivativeAssets $ 169,314us-gaap_DerivativeAssets
Liability Derivatives 133,236us-gaap_DerivativeLiabilities 187,399us-gaap_DerivativeLiabilities
Not Designated as Hedging Instrument [Member]    
Derivatives, Fair Value [Line Items]    
Derivatives, index options 114,287us-gaap_DerivativeAssets
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
169,314us-gaap_DerivativeAssets
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liability Derivatives 133,236us-gaap_DerivativeLiabilities
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
187,399us-gaap_DerivativeLiabilities
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Not Designated as Hedging Instrument [Member] | Equity Index Options [Member] | Derivatives, Index Options [Member]    
Derivatives, Fair Value [Line Items]    
Derivatives, index options 114,287us-gaap_DerivativeAssets
/ us-gaap_BalanceSheetLocationAxis
= nwli_DerivativesIndexOptionsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= nwli_EquityIndexOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
169,314us-gaap_DerivativeAssets
/ us-gaap_BalanceSheetLocationAxis
= nwli_DerivativesIndexOptionsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= nwli_EquityIndexOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Not Designated as Hedging Instrument [Member] | Fixed-Index Products [Member] | Universal Life and Annuity Contracts [Member]    
Derivatives, Fair Value [Line Items]    
Liability Derivatives $ 133,236us-gaap_DerivativeLiabilities
/ us-gaap_BalanceSheetLocationAxis
= nwli_UniversalLifeAndAnnuityContractsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= nwli_FixedIndexProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
$ 187,399us-gaap_DerivativeLiabilities
/ us-gaap_BalanceSheetLocationAxis
= nwli_UniversalLifeAndAnnuityContractsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= nwli_FixedIndexProductsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember