SCHEDULE IV - SUPPLEMENTARY REINSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 2,423,126	$ 2,112,178	$ 2,031,565
CEDED TO OTHER COMPANIES	1,529,130	1,217,234	929,064
ASSUMED FROM OTHER COMPANIES	3,133,147	2,858,030	2,572,165
NET AMOUNT	$ 4,027,143	$ 3,752,974	$ 3,674,666
PERCENTAGE OF AMOUNT ASSUMED TO NET	77.80%	76.20%	70.00%
Property and Casualty
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 2,228,022	$ 1,975,497	$ 1,901,757
CEDED TO OTHER COMPANIES	1,523,662	1,215,775	910,917
ASSUMED FROM OTHER COMPANIES	2,814,173	2,564,606	2,350,680
NET AMOUNT	$ 3,518,533	$ 3,324,328	$ 3,341,520
PERCENTAGE OF AMOUNT ASSUMED TO NET	80.00%	77.10%	70.30%
Accident and Health
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 195,104	$ 136,681	$ 129,808
CEDED TO OTHER COMPANIES	5,468	1,459	18,147
ASSUMED FROM OTHER COMPANIES	318,974	293,424	221,485
NET AMOUNT	$ 508,610	$ 428,646	$ 333,146
PERCENTAGE OF AMOUNT ASSUMED TO NET	62.70%	68.50%	66.50%