Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ (73)	$ (181)	$ (308)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(92)	(150)	(174)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(25)	(115)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	21	(4)	(9)
Corporate & Other
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ 0	$ (2)	$ (10)