Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (175)	$ (26)	$ (187)	$ 23
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	3	(11)	(30)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(180)	(23)	(198)	(7)
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(130)	(79)	(141)	(123)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(4)	(10)	(12)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(132)	(83)	(151)	(135)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(11)	71	0	179
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	0	(17)	(24)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(16)	71	(17)	155
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(34)	(17)	(46)	(32)
Pretax (favorable) unfavorable premium development, excluding Life & Group	2	7	16	6
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(32)	(10)	(30)	(26)
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	(1)	0	(1)
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0	0	0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 0	$ (1)	$ 0	$ (1)