Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (210)	$ (21)	$ (118)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(8)	(32)	(42)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(218)	(53)	(160)
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(141)	(136)	(196)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(11)	(13)	(14)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(152)	(149)	(210)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(15)	176	122
Pretax (favorable) unfavorable premium development, excluding Life & Group	(15)	(20)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(30)	156	114
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(54)	(59)	(38)
Pretax (favorable) unfavorable premium development, excluding Life & Group	18	2	(21)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(36)	(57)	(59)
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	(2)	(6)
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	(1)	1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 0	$ (3)	$ (5)