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Investments (Mortgage Loans and Real Estate) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount $ 144,568us-gaap_LoansReceivableCommercialRealEstate   $ 150,153us-gaap_LoansReceivableCommercialRealEstate
Allowance for possible losses, amount (650)us-gaap_AllowanceForLoanAndLeaseLossesRealEstate   (650)us-gaap_AllowanceForLoanAndLeaseLossesRealEstate
Totals, amount 143,918us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet [1]   149,503us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet [1]
Mortgage loans, percentage 100.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage   100.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
Allowance for possible losses, percentage (0.50%)nwli_MortgageLoansOnRealEstateAllowanceForPossibleLossPercentage   (0.40%)nwli_MortgageLoansOnRealEstateAllowanceForPossibleLossPercentage
Totals, percentage 99.50%nwli_MortgageLoansOnRealEstateByLoansToValueRatioPercentage [1]   99.60%nwli_MortgageLoansOnRealEstateByLoansToValueRatioPercentage [1]
Allowance for Loan and Lease Losses [Roll Forward]      
Balance, beginning of period 650us-gaap_LoansAndLeasesReceivableAllowance 650us-gaap_LoansAndLeasesReceivableAllowance  
Provision 0us-gaap_ProvisionForLoanAndLeaseLosses 0us-gaap_ProvisionForLoanAndLeaseLosses  
Releases 0us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet 0us-gaap_AllowanceForLoanAndLeaseLossesWriteoffsNet  
Balance, end of period 650us-gaap_LoansAndLeasesReceivableAllowance    
Less than 50% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount 47,448us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember		   52,564us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember
Mortgage loans, percentage 32.80%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember		   35.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioLessThanFiftyPercentMember
50% to 60% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount 49,388us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember		   50,553us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember
Mortgage loans, percentage 34.20%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember		   33.70%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioFiftyToSixtyPercentMember
60% to 70% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount 20,115us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember		   14,567us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember
Mortgage loans, percentage 13.90%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember		   9.70%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSixtyToSeventyPercentMember
70% to 80% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount 7,934us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember		   12,656us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember
Mortgage loans, percentage 5.50%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember		   8.40%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioSeventyToEightyPercentMember
80% to 90% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount 19,683us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember		   5,399us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember
Mortgage loans, percentage 13.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember		   3.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioEightyToNinetyPercentMember
Greater than 90% [Member]      
Mortgage Loans on Real Estate [Line Items]      
Mortgage loan, amount $ 0us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember		   $ 14,414us-gaap_LoansReceivableCommercialRealEstate
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember
Mortgage loans, percentage 0.00%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember		   9.60%nwli_MortgageLoansOnRealEstatePercentageOfTotalPercentage
/ nwli_MortgageLoansByLoanToValueRatioAxis
= nwli_MortgageLoansByLoansToValueRatioGreaterThanNinetyPercentMember
[1] Loan-to-Value Ratio determined using the most recent appraised value.