Fair Values of Financial Instruments (Fair Value by Balance Sheet Grouping) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
ASSETS
Debt securities held to maturity	$ 7,378,399		$ 7,407,703
Debt securities available for sale	3,410,656		3,356,945
Derivatives, index options	93,367		157,588
Equity securities	18,610		23,594
LIABILITIES
Contingent consideration on businesses acquired		$ 4,200
Restatement Adjustment
LIABILITIES
Increase in future policy benefits liability			15,000
Carrying Values
ASSETS
Debt securities held to maturity	6,822,053		7,106,245
Debt securities available for sale	3,410,656		3,356,945
Cash and cash equivalents	507,025		253,524
Mortgage loans	302,715		272,422
Real estate	33,927		34,588
Policy loans	75,714		80,008
Other loans	19,712		13,547
Derivatives, index options	93,367		157,588
Equity securities	18,610		23,594
Life interest in Libbie Shearn Moody Trust	9,230		9,230
Other investments	4,514		197
LIABILITIES
Deferred annuity contracts	6,697,569		7,014,833
Immediate annuity and supplemental contracts	417,663		400,465
Contingent consideration on businesses acquired	0		4,076
Fair Values
ASSETS
Debt securities held to maturity	7,378,399		7,407,703
Debt securities available for sale	3,410,656		3,356,945
Cash and cash equivalents	507,025		253,524
Mortgage loans	303,910		270,931
Real estate	48,547		57,204
Policy loans	128,615		123,650
Other loans	20,004		13,698
Derivatives, index options	93,367		157,588
Equity securities	18,610		23,594
Life interest in Libbie Shearn Moody Trust	12,775		12,775
Other investments	22,581		16,182
LIABILITIES
Deferred annuity contracts	5,422,599		5,931,352
Immediate annuity and supplemental contracts	449,676		422,931
Contingent consideration on businesses acquired	0		4,076
Level 1
ASSETS
Debt securities held to maturity	0		0
Debt securities available for sale	0		0
Cash and cash equivalents	507,025		253,524
Mortgage loans	0		0
Real estate	0		0
Policy loans	0		0
Other loans	0		0
Derivatives, index options	0		0
Equity securities	18,610		23,594
Life interest in Libbie Shearn Moody Trust	0		0
Other investments	1		2
LIABILITIES
Deferred annuity contracts	0		0
Immediate annuity and supplemental contracts	0		0
Contingent consideration on businesses acquired	0		0
Level 2
ASSETS
Debt securities held to maturity	7,378,399		7,407,703
Debt securities available for sale	3,410,656		3,356,945
Cash and cash equivalents	0		0
Mortgage loans	0		0
Real estate	0		0
Policy loans	0		0
Other loans	0		0
Derivatives, index options	0		0
Equity securities	0		0
Life interest in Libbie Shearn Moody Trust	0		0
Other investments	0		0
LIABILITIES
Deferred annuity contracts	0		0
Immediate annuity and supplemental contracts	0		0
Contingent consideration on businesses acquired	0		0
Level 3
ASSETS
Debt securities held to maturity	0		0
Debt securities available for sale	0		0
Cash and cash equivalents	0		0
Mortgage loans	303,910		270,931
Real estate	48,547		57,204
Policy loans	128,615		123,650
Other loans	20,004		13,698
Derivatives, index options	93,367		157,588
Equity securities	0		0
Life interest in Libbie Shearn Moody Trust	12,775		12,775
Other investments	22,580		16,180
LIABILITIES
Deferred annuity contracts	5,422,599		5,931,352
Immediate annuity and supplemental contracts	449,676		422,931
Contingent consideration on businesses acquired	$ 0		$ 4,076

Interest in Trust, Fair Value Disclosure

No definition available.

Name:	nwli_InterestInTrustFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Liability For Deferred Annuity Contracts, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_LiabilityForDeferredAnnuityContractsFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Liability for Future Policy Benefits, Individual and Group Annuities and Supplementary Contracts, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_LiabilityForFuturePolicyBenefitsIndividualAndGroupAnnuitiesAndSupplementaryContractsFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Loans, Gross, Insurance Policy, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_LoansGrossInsurancePolicyFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Mortgage Loans on Real Estate, Commercial and Consumer, Net, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_MortgageLoansOnRealEstateCommercialAndConsumerNetFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Other Assets, Book Value And Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_OtherAssetsBookValueAndFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Real Estate, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	nwli_RealEstateFairValueDisclosure
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26610-111562

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 35
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116859824&loc=d3e6819-128478

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

+ Details

Name:	us-gaap_BusinessCombinationContingentConsiderationLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

Name:	us-gaap_EquitySecuritiesFvNi
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=SL120269820-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before effect of reinsurance, of present value of future benefit to be paid to or on behalf of policyholder and related expense less present value of future net premium receivable under insurance contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121643868&loc=d3e14931-158439

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 55
-Paragraph 29E
-URI http://asc.fasb.org/extlink&oid=121639165&loc=SL117819541-158441

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(13)(a)(1))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (g)(1)
-URI http://asc.fasb.org/extlink&oid=121370832&loc=SL117420844-207641

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121643868&loc=d3e14931-158439

+ Details

Name:	us-gaap_LiabilityForFuturePolicyBenefits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

Name:	us-gaap_LoansReceivableFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_RestatementAxis=srt_RestatementAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: