SCHEDULE IV - SUPPLEMENTARY REINSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 3,468,040	$ 2,423,126	$ 2,112,178
CEDED TO OTHER COMPANIES	2,251,103	1,529,130	1,217,234
ASSUMED FROM OTHER COMPANIES	3,442,025	3,133,147	2,858,030
NET AMOUNT	$ 4,658,962	$ 4,027,143	$ 3,752,974
PERCENTAGE OF AMOUNT ASSUMED TO NET	73.90%	77.80%	76.20%
Property and Casualty
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 3,258,999	$ 2,228,022	$ 1,975,497
CEDED TO OTHER COMPANIES	2,163,417	1,523,662	1,215,775
ASSUMED FROM OTHER COMPANIES	3,074,906	2,814,173	2,564,606
NET AMOUNT	$ 4,170,488	$ 3,518,533	$ 3,324,328
PERCENTAGE OF AMOUNT ASSUMED TO NET	73.70%	80.00%	77.10%
Accident and health
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 209,041	$ 195,104	$ 136,681
CEDED TO OTHER COMPANIES	87,686	5,468	1,459
ASSUMED FROM OTHER COMPANIES	367,119	318,974	293,424
NET AMOUNT	$ 488,474	$ 508,610	$ 428,646
PERCENTAGE OF AMOUNT ASSUMED TO NET	75.20%	62.70%	68.50%