Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (132)	$ (175)	$ (282)	$ (187)
Commercial [Member]
Commercial Auto	(12)	0	(47)	7
General Liability	14	3	(38)	8
Workers' Compensation	(6)	(1)	48	22
Property and Other	(1)	(13)	0	(37)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (5)	$ (11)	$ (37)	$ 0

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

No definition available.

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

No definition available.

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

No definition available.

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

No definition available.

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

No definition available.

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