Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (132)	$ (175)	$ (282)	$ (187)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(5)	(27)	(11)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(137)	(180)	(309)	(198)
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(112)	(130)	(211)	(141)
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	(2)	(18)	(10)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(112)	(132)	(229)	(151)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(5)	(11)	(37)	0
Pretax (favorable) unfavorable premium development, excluding Life & Group	(3)	(5)	(7)	(17)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(8)	(16)	(44)	(17)
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)	(34)	(34)	(46)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	2	(2)	16
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(17)	(32)	(36)	(30)
Corporate and Other Non-Core [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