Investments (Narrative) (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021 USD ($) item loan	Dec. 31, 2020 USD ($) item loan
Schedule of Investments [Line Items]
Decrease in gross AFS securities unrealized gains (losses)	$ 188
Number of partnerships in alternative investment portfolio \| item	297	270
Alternative investments as a percentage of overall invested assets	2.00%	1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 23
Investment commitments	2,700
Investment commitments for limited partnerships	1,700
Investment commitments for private placements	346
Investment commitments for mortgage loans on real estate	$ 653
Minimum [Member]
Schedule of Investments [Line Items]
Percentage of the fair value of securities obtained as collateral under reverse repurchase agreements.	80.00%
Maximum [Member]
Schedule of Investments [Line Items]
Percentage of the fair value of securities obtained as collateral under reverse repurchase agreements.	95.00%
Commercial [Member]
Schedule of Investments [Line Items]
Number of impaired loans \| item	6	4
Impaired financing receivable, carrying amount	$ 1	$ 1
Residential [Member]
Schedule of Investments [Line Items]
Number of loans past due \| loan	65	147
Number of impaired loans	54	76
Impaired financing receivable, carrying amount	$ 25	$ 34
Federal Home Loan Mortgage Corporation [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Fair value	1,100	1,200
White Chapel LLC [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Fair value		1,000
Fannie Mae [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Fair value	1,000
Financial Service [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Fair value	21,900	21,700
Consumer Non-Cyclical Industry [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Fair value	$ 19,000	$ 19,200
Mortgage Loans On Real Estate [Member] \| Geographic Concentration [Member] \| Commercial [Member] \| California [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	26.00%	24.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration [Member] \| Commercial [Member] \| Texas [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	9.00%	10.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration [Member] \| Residential [Member] \| California [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	23.00%	32.00%
Mortgage Loans On Real Estate [Member] \| Geographic Concentration [Member] \| Residential [Member] \| Florida [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	16.00%	18.00%
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 3,600	$ 3,800
Fair value of portfolio rated below investment grade	$ 3,800	$ 4,000
Fixed Maturity AFS Securities [Member] \| Federal Home Loan Mortgage Corporation [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	1.00%	1.00%
Fixed Maturity AFS Securities [Member] \| White Chapel LLC [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage		1.00%
Fixed Maturity AFS Securities [Member] \| Fannie Mae [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	1.00%
Fixed Maturity AFS Securities [Member] \| Financial Service [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	14.00%	14.00%
Fixed Maturity AFS Securities [Member] \| Consumer Non-Cyclical Industry [Member] \| Investments [Member]
Schedule of Investments [Line Items]
Concentration risk, percentage	12.00%	13.00%

Alternative investments as a percentage of overall invested assets.

No definition available.

Name:	cik0000726865_AlternativeInvestmentsAsPercentageOfOverallInvestedAssets
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

This item represents the percentage of the fair value of securities obtained as collateral under reverse repurchase agreements

+ References

No definition available.

+ Details

Name:	cik0000726865_CollateralObtainedPercentOfFairValueReverseRepurchaseAgreements
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

Financing receivable, Number of loans past due

+ References

No definition available.

+ Details

Name:	cik0000726865_FinancingReceivableNumberOfLoansPastDue
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Impaired financing receivable, number of impaired loans

+ References

No definition available.

+ Details

Name:	cik0000726865_ImpairedFinancingReceivableNumberOfImpairedLoans
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

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

Number of partnerships represented in alternative investment portfolio.

+ References

No definition available.

+ Details

Name:	cik0000726865_NumberOfPartnershipsInAlternativeInvestmentPortfolio
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:positiveIntegerItemType
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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

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

- Definition

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13531-108611

+ 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953550-111524

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_FederalHomeLoanMortgageCorporationCertificatesAndObligationsFHLMCMember
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=cik0000726865_WhiteChapelLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=cik0000726865_GeographicConcentrationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_TX
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_FL
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=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: