Share-Based Payments - Exercise Range (Details) $ / shares in Units, $ in Thousands	6 Months Ended
Jun. 30, 2023 USD ($) $ / shares shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number Outstanding (in shares)	281,820
Number Exercisable (in shares)	117,981
SARs
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number Outstanding, Aggregate intrinsic value \| $	$ 53,494
Number Exercisable, Aggregate intrinsic value \| $	$ 21,120
SARs \| 210.22
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 210.22
Number Outstanding (in shares)	21,750
Weighted- Average Remaining Contractual Life	4 months 24 days
Number Exercisable (in shares)	21,750
SARs \| 216.48
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 216.48
Number Outstanding (in shares)	10,342
Weighted- Average Remaining Contractual Life	2 years 7 months 6 days
Number Exercisable (in shares)	10,342
SARs \| 311.16
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 311.16
Number Outstanding (in shares)	7,881
Weighted- Average Remaining Contractual Life	3 years 7 months 6 days
Number Exercisable (in shares)	7,881
SARs \| 334.34
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 334.34
Number Outstanding (in shares)	7,443
Weighted- Average Remaining Contractual Life	4 years 6 months
Number Exercisable (in shares)	7,443
SARs \| 303.77
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 303.77
Number Outstanding (in shares)	9,357
Weighted- Average Remaining Contractual Life	5 years 6 months
Number Exercisable (in shares)	9,357
SARs \| 252.91
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 252.91
Number Outstanding (in shares)	16,958
Weighted- Average Remaining Contractual Life	6 years 6 months
Number Exercisable (in shares)	16,958
SARs \| 192.1
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 192.10
Number Outstanding (in shares)	36,503
Weighted- Average Remaining Contractual Life	7 years 4 months 24 days
Number Exercisable (in shares)	24,327
SARs \| 218.44
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 218.44
Number Outstanding (in shares)	59,802
Weighted- Average Remaining Contractual Life	8 years 6 months
Number Exercisable (in shares)	19,923
SARs \| 220.61
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price (in dollars per share) \| $ / shares	$ 220.61
Number Outstanding (in shares)	111,784
Weighted- Average Remaining Contractual Life	9 years 6 months
Number Exercisable (in shares)	0

Share-Based Payment Arrangement, Option, Exercise Price Range

No definition available.

Name:	nwli_ShareBasedPaymentArrangementOptionExercisePriceRange
Namespace Prefix:	nwli_
Data Type:	dtr-types1:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

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

- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(iii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

+ References

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 718
-SubTopic 10
-Subparagraph (e)(2)
-Name Accounting Standards Codification
-Paragraph 2
-Section 50
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_StockAppreciationRightsSARSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange6Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange7Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange8Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange9Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=nwli_ExercisePriceRange10Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: