Schedule IV (SUPPLEMENTAL INFORMATION CONCERNING REINSURANCE) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 35,876	$ 34,108	$ 32,782
Ceded To Other Companies									7,693	7,219	7,080
Assumed From Other Companies									3,107	3,175	3,332
Net Amount	$ 7,935	$ 8,327	$ 7,891	$ 7,137	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 31,290	$ 30,064	$ 29,034
Percentage of Amount Assumed to Net									10.00%	11.00%	11.00%
Property and Casualty [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 30,339	$ 28,793	$ 27,774
Ceded To Other Companies									7,236	6,792	6,650
Assumed From Other Companies									2,797	2,812	2,891
Net Amount									$ 25,900	$ 24,813	$ 24,015
Percentage of Amount Assumed to Net									11.00%	11.00%	12.00%
Accident and Health [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 4,546	$ 4,409	$ 4,167
Ceded To Other Companies									376	342	349
Assumed From Other Companies									119	162	221
Net Amount									$ 4,289	$ 4,229	$ 4,039
Percentage of Amount Assumed to Net									3.00%	4.00%	5.00%
Life [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 991	$ 906	$ 841
Ceded To Other Companies									81	85	81
Assumed From Other Companies									191	201	220
Net Amount									$ 1,101	$ 1,022	$ 980
Percentage of Amount Assumed to Net									17.00%	20.00%	22.00%