SCHEDULE IV - SUPPLEMENTARY REINSURANCE INFORMATION (DETAILS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 2,112,178	$ 2,031,565	$ 2,009,974
CEDED TO OTHER COMPANIES	1,217,234	929,064	804,544
ASSUMED FROM OTHER COMPANIES	2,858,030	2,572,165	2,701,545
NET AMOUNT	$ 3,752,974	$ 3,674,666	$ 3,906,975
PERCENTAGE OF AMOUNT ASSUMED TO NET	76.20%	70.00%	69.10%
Property and Casualty [Member]
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 1,975,497	$ 1,901,757	$ 1,882,695
CEDED TO OTHER COMPANIES	1,215,775	910,917	786,555
ASSUMED FROM OTHER COMPANIES	2,564,606	2,350,680	2,548,174
NET AMOUNT	$ 3,324,328	$ 3,341,520	$ 3,644,314
PERCENTAGE OF AMOUNT ASSUMED TO NET	77.10%	70.30%	69.90%
Accident and Health [Member]
Supplementary Insurance Information [Line Items]
DIRECT GROSS PREMIUM	$ 136,681	$ 129,808	$ 127,279
CEDED TO OTHER COMPANIES	1,459	18,147	17,989
ASSUMED FROM OTHER COMPANIES	293,424	221,485	153,371
NET AMOUNT	$ 428,646	$ 333,146	$ 262,661
PERCENTAGE OF AMOUNT ASSUMED TO NET	68.50%	66.50%	58.40%