Future Contract Benefits - Summary of Discounted and Undiscounted Expected Future Gross premiums and Expected Future Benefit Payments (Details) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Payout Annuities
Undiscounted
Expected future gross premiums	$ 0	$ 0
Expected future benefit payments	3,242	3,407
Discounted
Expected future gross premiums	0	0
Expected future benefit payments	1,990	2,032
Traditional Life
Undiscounted
Expected future gross premiums	12,601	13,342
Expected future benefit payments	12,559	13,729
Discounted
Expected future gross premiums	8,663	9,091
Expected future benefit payments	$ 8,636	$ 9,312

Liability For Future Policy Benefit, Discounted, Befiore Reinsurance

No definition available.

Name:	cik0000726865_LiabilityForFuturePolicyBenefitDiscountedBefioreReinsuranceAbstract
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Liability for future policy benefit, expected future policy benefit, discounted, before reinsurance.

+ References

No definition available.

+ Details

Name:	cik0000726865_LiabilityForFuturePolicyBenefitExpectedFuturePolicyBenefitDiscountedBeforeReinsurance
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Liability For Future Policy Benefit, Undiscounted, Befiore Reinsurance

+ References

No definition available.

+ Details

Name:	cik0000726865_LiabilityForFuturePolicyBenefitUndiscountedBefioreReinsuranceAbstract
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before effect of reinsurance, of discounted balance for expected future gross premium component of liability for future policy benefit.

+ References

+ Details

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

- Definition

Amount, before effect of reinsurance, of undiscounted balance for expected future gross premium component of liability for future policy benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 29E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-29E

+ Details

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

- Definition

Amount, before effect of reinsurance, of undiscounted balance for expected future policy benefit and expense component of liability for future policy benefit.

+ References

+ Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cik0000726865_PayoutAnnuitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cik0000726865_TraditionalLifeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: