Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (52)	$ (7)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(14)	9
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(66)	2
Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(34)	2
Pretax (favorable) unfavorable premium development, excluding Life & Group	(11)	(6)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(45)	(4)
Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(14)	(5)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(16)	(6)
International [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(4)	(4)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(1)	16
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(5)	12
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
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 0	$ 0