Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (98)	$ (5)	$ (150)	$ (12)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(8)	(15)	(22)	(6)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(106)	(20)	(172)	(18)
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(65)	(13)	(99)	(11)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(7)	(2)	(18)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(72)	(15)	(117)	(19)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(18)	16	(32)	11
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(11)	(4)	(12)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(20)	5	(36)	(1)
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(15)	(8)	(19)	(12)
Pretax (favorable) unfavorable premium development, excluding Life & Group	1	(2)	0	14
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(14)	(10)	(19)	2
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