SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	$ 566,622	$ 474,061	$ 438,636
Reserve for losses and loss expenses	12,280,769	12,997,553	9,697,827
Unearned premiums	3,635,758	3,641,399	2,969,498
Net premiums earned	4,791,495	4,148,760	3,705,625
Net Investment Income	438,507	400,805	353,335
Net losses and loss expenses	3,190,287	3,287,772	2,204,197
Acquisition costs	968,835	823,591	746,876
Other Operating Expenses	627,389	579,428	602,717
Net premiums written	4,658,962	4,027,143	3,752,974
Operating Segments | Insurance
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	209,622	115,332	100,149
Reserve for losses and loss expenses	6,426,309	7,011,805	5,198,070
Unearned premiums	2,061,123	2,053,422	1,423,891
Net premiums earned	2,362,606	1,816,438	1,534,282
Net Investment Income	0	0	0
Net losses and loss expenses	1,494,323	1,465,427	977,771
Acquisition costs	399,193	270,229	206,619
Other Operating Expenses	395,252	325,368	327,351
Net premiums written	2,324,747	1,775,825	1,519,559
Operating Segments | Reinsurance
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	357,000	358,729	338,487
Reserve for losses and loss expenses	5,854,460	5,985,748	4,499,757
Unearned premiums	1,574,635	1,587,977	1,545,607
Net premiums earned	2,428,889	2,332,322	2,171,343
Net Investment Income	0	0	0
Net losses and loss expenses	1,695,964	1,822,345	1,226,426
Acquisition costs	569,642	553,362	540,257
Other Operating Expenses	123,916	124,115	155,350
Net premiums written	2,334,215	2,251,318	2,233,415
Corporate
Supplementary Insurance Information [Line Items]
Deferred acquisition costs	0	0	0
Reserve for losses and loss expenses	0	0	0
Unearned premiums	0	0	0
Net premiums earned	0	0	0
Net Investment Income	438,507	400,805	353,335
Net losses and loss expenses	0	0	0
Acquisition costs	0	0	0
Other Operating Expenses	108,221	129,945	120,016
Net premiums written	$ 0	$ 0	$ 0