Pension and Other Postretirement Plans - Narrative (Details) $ in Millions	3 Months Ended
Mar. 31, 2022 USD ($) benefit_plan	Dec. 31, 2007
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of non-qualified defined benefit pension plans \| benefit_plan	3
Number of healthcare plans \| benefit_plan	2
Defined Benefit Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Vesting percentage in accrued benefits from plan freeze		100.00%
Minimum required contribution	$ 0.0
Planned contributions remaining	0.3
Maximum contributions payable	0.3
Company contributions to 2021 year plan	0.0
Company contributions to plan	0.0
Chairman and President Non-Qualified Defined Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Company contributions to plan	$ 0.4
Plan participant's age threshold for Company's contingent liability with respect to the Plan	70 years
Aggregate average annual participant salary increase	10.00%
Company expected contributions to plans in fiscal year	$ 2.0

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Participant Age

No definition available.

Name:	nwli_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostParticipantAge
Namespace Prefix:	nwli_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan, Minimum Required Employer Contributions For Current Plan Year

+ References

No definition available.

+ Details

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

- Definition

Defined Benefit Plan, Number Of Non-Qualified Defined Benefit Pension Plan

+ References

No definition available.

+ Details

Name:	nwli_DefinedBenefitPlanNumberOfNonQualifiedDefinedBenefitPensionPlan
Namespace Prefix:	nwli_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Defined Benefit Plan, Plan Assets, Contributions by Employer, Prior Year

+ References

No definition available.

+ Details

Name:	nwli_DefinedBenefitPlanPlanAssetsContributionsByEmployerPriorYear
Namespace Prefix:	nwli_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Defined Benefit Plan, Plan Assets, Contributions Payable

+ References

No definition available.

+ Details

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

- Definition

Defined Benefit Plans, Number of Healthcare Plans

+ References

No definition available.

+ Details

Name:	nwli_DefinedBenefitPlansNumberOfHealthcarePlans
Namespace Prefix:	nwli_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Defined Benefit Plans, Vesting Percentage in Accrued Benefits

+ References

No definition available.

+ Details

Name:	nwli_DefinedBenefitPlansVestingPercentageInAccruedBenefits
Namespace Prefix:	nwli_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of contributions received and expected to be received by defined benefit plan from employer in current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

+ Details

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

- Definition

Amount of contribution expected to be received by defined benefit plan from employer in remainder of current fiscal year. Excludes contribution previously paid by employer in current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e2709-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: