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Investments (Mortgage Loans) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount $ 150,153us-gaap_LoansReceivableCommercialRealEstate $ 133,415us-gaap_LoansReceivableCommercialRealEstate
Allowance for possible losses, amount (650)us-gaap_AllowanceForLoanAndLeaseLossesRealEstate (650)us-gaap_AllowanceForLoanAndLeaseLossesRealEstate
Totals, amount 149,503us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet 132,765us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Mortgage loans, percentage 100.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage 100.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
Allowance for possible losses, percentage (0.40%)nwli_MortgageLoansOnRealEstateAllowanceForPossibleLossPercentage (0.50%)nwli_MortgageLoansOnRealEstateAllowanceForPossibleLossPercentage
Totals, percentage 99.60%nwli_MortgageLoansOnRealEstateNetPercentage 99.50%nwli_MortgageLoansOnRealEstateNetPercentage
Less than 50% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 52,564us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember		 57,445us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember
Mortgage loans, percentage 35.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember		 43.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember
50% to 60% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 50,553us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember		 23,339us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember
Mortgage loans, percentage 33.70%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember		 17.50%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember
60% to 70% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 14,567us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember		 20,120us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember
Mortgage loans, percentage 9.70%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember		 15.10%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember
70% to 80% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 12,656us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember		 9,723us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember
Mortgage loans, percentage 8.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember		 7.30%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember
80% to 90% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 5,399us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember		 0us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember
Mortgage loans, percentage 3.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember		 0.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember
Greater than 90% [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 14,414us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember		 22,788us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember
Mortgage loans, percentage 9.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember		 17.10%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember
Retail [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 130,544us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_RetailMember		 103,487us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_RetailMember
Mortgage loans, percentage 86.90%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_RetailMember		 77.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_RetailMember
Apartment [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 7,333us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_ApartmentMember		 15,275us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_ApartmentMember
Mortgage loans, percentage 4.90%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_ApartmentMember		 11.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_ApartmentMember
Land and Lots [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 3,333us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_LandAndLotsMember		 3,155us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_LandAndLotsMember
Mortgage loans, percentage 2.20%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_LandAndLotsMember		 2.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_LandAndLotsMember
Hotal and Motel [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 1,600us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_HotalAndMotelMember		 3,742us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_HotalAndMotelMember
Mortgage loans, percentage 1.10%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_HotalAndMotelMember		 2.80%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_HotalAndMotelMember
Office [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 893us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_OfficeMember		 1,313us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_OfficeMember
Mortgage loans, percentage 0.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_OfficeMember		 1.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_OfficeMember
All Other [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 6,450us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_AllOtherMember		 6,443us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_AllOtherMember
Mortgage loans, percentage 4.30%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_AllOtherMember		 4.80%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortagageLoanByPropertyTypeAxis
= nwli_AllOtherMember
West South Central [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 97,918us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_WestSouthCentralMember		 79,702us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_WestSouthCentralMember
Mortgage loans, percentage 65.20%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_WestSouthCentralMember		 59.70%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_WestSouthCentralMember
New England [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 12,155us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_NewEnglandMember		 17,400us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_NewEnglandMember
Mortgage loans, percentage 8.10%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_NewEnglandMember		 13.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_NewEnglandMember
Mountain [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 3,050us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_MountainMember		 3,239us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_MountainMember
Mortgage loans, percentage 2.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_MountainMember		 2.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_MountainMember
Pacific [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 10,282us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_PacificMember		 10,364us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_PacificMember
Mortgage loans, percentage 6.90%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_PacificMember		 7.80%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_PacificMember
East South Central [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 14,137us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_EastSouthCentralMember		 10,098us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_EastSouthCentralMember
Mortgage loans, percentage 9.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_EastSouthCentralMember		 7.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_EastSouthCentralMember
East North Central [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount 10,714us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_EastNorthCentralMember		 10,607us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_EastNorthCentralMember
Mortgage loans, percentage 7.10%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_EastNorthCentralMember		 8.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_EastNorthCentralMember
Middle Atlantic [Member]    
Mortgage Loans on Real Estate [Line Items]    
Mortgage loans, amount $ 1,897us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_MiddleAtlanticMember		 $ 2,005us-gaap_LoansReceivableCommercialRealEstate
/ us-gaap_StatementGeographicalAxis
= nwli_MiddleAtlanticMember
Mortgage loans, percentage 1.30%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_MiddleAtlanticMember		 1.50%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ us-gaap_StatementGeographicalAxis
= nwli_MiddleAtlanticMember