Schedule IV (SUPPLEMENTAL INFORMATION CONCERNING REINSURANCE) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 37,037	$ 35,876	$ 34,108
Ceded To Other Companies	7,243	7,693	7,219
Assumed From Other Companies	3,323	3,107	3,175
Net Amount	$ 33,117	$ 31,290	$ 30,064
Percentage of Amount Assumed to Net	10.00%	10.00%	11.00%
Property and Casualty [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 31,546	$ 30,339	$ 28,793
Ceded To Other Companies	6,782	7,236	6,792
Assumed From Other Companies	3,044	2,797	2,812
Net Amount	$ 27,808	$ 25,900	$ 24,813
Percentage of Amount Assumed to Net	11.00%	11.00%	11.00%
Accident and Health [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 4,249	$ 4,546	$ 4,409
Ceded To Other Companies	368	376	342
Assumed From Other Companies	111	119	162
Net Amount	$ 3,992	$ 4,289	$ 4,229
Percentage of Amount Assumed to Net	3.00%	3.00%	4.00%
Life [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount	$ 1,242	$ 991	$ 906
Ceded To Other Companies	93	81	85
Assumed From Other Companies	168	191	201
Net Amount	$ 1,317	$ 1,101	$ 1,022
Percentage of Amount Assumed to Net	13.00%	17.00%	20.00%