Alternative investments as a percentage of overall invested assets.

No definition available.

Name:	cik0000726865_AlternativeInvestmentsAsAPercentageOfOverallInvestedAssets
Namespace Prefix:	cik0000726865_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost of portfolio rated below investment grade.

+ References

No definition available.

+ Details

Name:	cik0000726865_AmortizedCostOfPortfolioRatedBelowInvestmentGrade
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the change in gross unrealized holding gain or loss on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

+ References

No definition available.

+ Details

Name:	cik0000726865_AvailableForSaleSecuritiesChangeInGrossUnrealizedHoldingGainLoss
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Percentage of the fair value of portfolio that was rated investment grade as of balance sheet date.

+ References

No definition available.

+ Details

Name:	cik0000726865_AvailableForSaleSecuritiesFairValueDisclosurePercentInvestmentGrade
Namespace Prefix:	cik0000726865_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value of portfolio rated below investment grade.

+ References

No definition available.

+ Details

Name:	cik0000726865_FairValueOfPortfolioRatedBelowInvestmentGrade
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents total investment commitments as of the balance sheet date for fixed maturity available-for-sale securities, limited partnerships, real estate and mortgage loans on real estate.

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentCommitments
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Investment Commitments, Asset-Backed Variable Interest Entities

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentCommitmentsAssetBackedVariableInterestEntities
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents investment commitments as of the balance sheet date for limited partnerships.

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentCommitmentsLimitedPartnerships
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents investment commitments as of the balance sheet date for private placements.

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentCommitmentsLimitedPrivatePlacements
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents investment commitments as of the balance sheet date for mortgage loans.

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentCommitmentsMortgageLoans
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Investments

+ References

No definition available.

+ Details

Name:	cik0000726865_InvestmentsLineItems
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of partnerships represented in alternative investment portfolio.

+ References

No definition available.

+ Details

Name:	cik0000726865_NumberOfPartnershipsInAlternativeInvestmentPortfolio
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482810/280-10-50-42

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 20
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-20

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 275
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482861/275-10-50-18

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of investment subject to concentration of credit or market risk or both.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-20

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-21

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 23
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-23

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 23
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-23

+ Details

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

- Definition

Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-1A

+ Details

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

- Definition

Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-1A

+ Details

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

- Definition

Amortized cost of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_CounterpartyNameAxis=cik0000726865_DerivativeCounterpartiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=cik0000726865_WhiteChapelVLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=cik0000726865_WhiteChapelLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FinancialServiceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_SecuritiesInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=cik0000726865_ConsumerNonCyclicalIndustryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=cik0000726865_CreditAssessmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_ReinsurerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=cik0000726865_InvestedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_LenderConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=cik0000726865_WhiteChapelVAndWhiteChapelLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments - Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
Investments - Narrative (Details) $ in Millions	Sep. 30, 2025 USD ($) investment	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) investment	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($) investment
Investments [Line Items]
Decrease in gross AFS securities unrealized losses			$ 1,600
Losses from loan modifications	$ 3	$ 1	$ 21	$ 3
Number of partnerships in alternative investment portfolio \| investment	362		362		351
Alternative investments as a percentage of overall invested assets			3.00%		3.00%
Investment commitments	$ 5,900		$ 5,900
Investment commitments for limited partnerships	3,600		3,600
Investment commitments for mortgage loans on real estate	1,400		1,400
Investment commitments, asset-backed interest entities	600		600
Investment commitments for private placements	351		351
Derivative Counterparties
Investments [Line Items]
Fair value of collateral received that we are permitted to sell or re-pledge	1,600		1,600
Fair value of securities received as collateral that have been resold or repledged	593		593
Corporate bonds
Investments [Line Items]
Amortized cost of portfolio rated below investment grade	2,800		2,800		$ 2,700
Fair value of portfolio rated below investment grade	2,700		2,700		2,700
White Chapel V LLC \| CMBS
Investments [Line Items]
Fair value, concentration of risk	1,400		1,400		1,500
White Chapel LLC \| CMBS
Investments [Line Items]
Fair value, concentration of risk	1,100		1,100		1,100
Financial Service \| Securities Investment
Investments [Line Items]
Fair value, concentration of risk	16,500		16,500		16,200
Consumer Non-Cyclical Industry \| Securities Investment
Investments [Line Items]
Fair value, concentration of risk	$ 10,900		$ 10,900		$ 10,700
Credit Assessment \| Reinsurer Concentration Risk \| Corporate bonds
Investments [Line Items]
Percentage of fair value rated as investment grade	96.00%		96.00%		96.00%
Investments \| Lender Concentration Risk \| White Chapel V And White Chapel LLC \| CMBS
Investments [Line Items]
Concentration risk			1.00%		1.00%
Investments \| Lender Concentration Risk \| Financial Service \| Securities Investment
Investments [Line Items]
Concentration risk			13.00%		13.00%
Investments \| Lender Concentration Risk \| Consumer Non-Cyclical Industry \| Securities Investment
Investments [Line Items]
Concentration risk			8.00%		9.00%