Schedule IV (SUPPLEMENTAL INFORMATION CONCERNING REINSURANCE) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 34,108	$ 32,782	$ 31,811
Ceded To Other Companies									7,219	7,080	6,806
Assumed From Other Companies									3,175	3,332	3,744
Net Amount	$ 7,465	$ 7,908	$ 7,664	$ 7,027	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 30,064	$ 29,034	$ 28,749
Percentage of Amount Assumed to Net									11.00%	11.00%	13.00%
Property and Casualty [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 28,793	$ 27,774	$ 26,919
Ceded To Other Companies									6,792	6,650	6,407
Assumed From Other Companies									2,812	2,891	3,284
Net Amount									$ 24,813	$ 24,015	$ 23,796
Percentage of Amount Assumed to Net									11.00%	12.00%	14.00%
Accident and Health [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 4,409	$ 4,167	$ 4,047
Ceded To Other Companies									342	349	315
Assumed From Other Companies									162	221	219
Net Amount									$ 4,229	$ 4,039	$ 3,951
Percentage of Amount Assumed to Net									4.00%	5.00%	6.00%
Life [Member]
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Amount									$ 906	$ 841	$ 845
Ceded To Other Companies									85	81	84
Assumed From Other Companies									201	220	241
Net Amount									$ 1,022	$ 980	$ 1,002
Percentage of Amount Assumed to Net									20.00%	22.00%	24.00%