Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ (20)	$ (73)	$ (181)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(61)	(92)	(150)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	43	(2)	(25)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	21	(4)
Corporate & Other
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable premium development, excluding Life & Group	$ 0	$ 0	$ (2)