Fair Value of Financial Instruments - Changes in Unrealized Gains (Losses) Included in OCI (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	$ (48)	$ (4)	$ (55)	$ 18
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Unrealized investment gain (loss)	Unrealized investment gain (loss)	Unrealized investment gain (loss)	Unrealized investment gain (loss)
Corporate bonds
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	$ (27)	$ 0	$ (37)	$ 12
State and municipal bonds
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	0	3	0	3
ABS
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	(20)	(7)	(20)	1
Hybrid and redeemable preferred securities
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	(1)	(2)	1	(2)
Mortgage loans on real estate
Fair Value Assets and Liabilities Measured on Recurring Basis Level 3 Activity [Line Items]
Unrealized gains (losses) included in OCI, net	$ 0	$ 2	$ 1	$ 4

Fair Value Assets And Liabilities Measured On Recurring Basis Level 3 Activity [Line Items]

No definition available.

Name:	cik0000726865_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisLevel3ActivityLineItems
Namespace Prefix:	cik0000726865_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_FairValueAssetRecurringBasisStillHeldUnrealizedGainLossOci
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Indicates line item in statement in which other comprehensive income (OCI) is reported that includes unrealized gain (loss) from asset measured at fair value using unobservable input (level 3) and still held.

+ References

+ Details

Name:	us-gaap_FairValueAssetRecurringBasisStillHeldUnrealizedGainLossStatementOfOtherComprehensiveIncomeExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=cik0000726865_HybridAndRedeemablePreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=cik0000726865_MortgageLoansOnRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: