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Claim and Claim Adjustment Expense Reserves, Net Prior Year Development in Property and Casualty Operations (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | $ (15) | $ (14)
Medical Professional Liability [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | 10 | 15
Other Professional Liability and Management Liability [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | 3 | (12)
Surety [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | (30) | (25)
Commercial Auto [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | 9 | (5)
General Liability [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development |  | (20)
Workers' Compensation [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | (13) | 2
Property and Other [Member] |  |
Net Prior Year Development [Abstract] |  |
Pretax (favorable) unfavorable development | $ 6 | $ 31

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- Definition Amount of pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development. + References No definition available. + Details
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Namespace Prefix: | l_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |