Claim and Claim Adjustment Expense Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ 0	$ 9
General Liability	0	0
Workers' Compensation	0	(13)
Property and Other	0	0
Total pretax (favorable) unfavorable development	$ 0	$ (4)

Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto

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- Definition

Net prior year claim and allocated claim adjustment expense reserve development, General Liability

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- Definition

Net prior year claim and allocated claim adjustment expense reserve development, Property and Other

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- Definition

Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation

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- Definition

Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group

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- 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.

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