Schedule IV (SUPPLEMENTAL INFORMATION CONCERNING REINSURANCE) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Percentage of Amount Assumed to Net									11.00%	13.00%	21.00%
Net Amount	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 29,034	$ 28,749	$ 17,213
Assumed From Other Companies									3,332	3,744	3,676
Ceded To Other Companies									7,080	6,806	5,818
Direct Amount									$ 32,782	$ 31,811	$ 19,355
Property and Casualty [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Percentage of Amount Assumed to Net									12.00%	14.00%	25.00%
Net Amount									$ 24,015	$ 23,796	$ 12,781
Assumed From Other Companies									2,891	3,284	3,259
Ceded To Other Companies									6,650	6,407	5,373
Direct Amount									$ 27,774	$ 26,919	$ 14,895
Accident and Health [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Percentage of Amount Assumed to Net									5.00%	6.00%	5.00%
Net Amount									$ 4,039	$ 3,951	$ 3,501
Assumed From Other Companies									221	219	168
Ceded To Other Companies									349	315	351
Direct Amount									$ 4,167	$ 4,047	$ 3,684
Life [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Percentage of Amount Assumed to Net									22.00%	24.00%	27.00%
Net Amount									$ 980	$ 1,002	$ 931
Assumed From Other Companies									220	241	249
Ceded To Other Companies									81	84	94
Direct Amount									$ 841	$ 845	$ 776