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	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (98)	$ (5)	$ (150)	$ (12)
Commercial [Member]
Commercial Auto	(20)	7	(35)	7
General Liability	(37)	1	(52)	5
Workers' Compensation	50	24	54	23
Property and Other	(11)	(16)	1	(24)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (18)	$ 16	$ (32)	$ 11

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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