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Pension and Other Postretirement Plans (Contributions) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Defined Benefit Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected contribution for next fiscal year $ 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Defined Benefit Pension Plans [Member] | Minimum [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Adjusted funding target attainment percentage 80.00%nwli_AdjustedFundingTargetAttainmentPercentage
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Chairman and President Non-Qualified Defined Benefit Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected contribution for next fiscal year $ 2,000us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember