Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (5)	$ 24	$ (12)	$ 49
Pretax (favorable) unfavorable premium development, excluding Life & Group	(15)	(2)	(6)	(33)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(20)	22	(18)	16
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(13)	(41)	(11)	(44)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(2)	(8)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(15)	(43)	(19)	(52)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	16	90	11	108
Pretax (favorable) unfavorable premium development, excluding Life & Group	(11)	(6)	(12)	(24)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	5	84	(1)	84
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(8)	(25)	(12)	(15)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	6	14	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(10)	(19)	2	(16)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	0	0	0	0
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0	0	0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 0	$ 0	$ 0	$ 0