Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (288)	$ (210)	$ (21)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(26)	(8)	(32)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(314)	(218)	(53)
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(287)	(141)	(136)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(18)	(11)	(13)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(305)	(152)	(149)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	55	(15)	176
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(15)	(20)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	53	(30)	156
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(58)	(54)	(59)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(6)	18	2
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(64)	(36)	(57)
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	2	0	(2)
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 2	$ 0	$ (3)