SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	$ 474,061	$ 438,636	$ 471,782
Reserve for Losses and Loss Expenses	12,997,553	9,697,827	9,646,285
Unearned Premiums	3,641,399	2,969,498	2,760,889
Net Premiums Earned	4,148,760	3,705,625	3,686,417
Net Investment Income	400,805	353,335	305,336
Losses And Loss Expenses	3,287,772	2,204,197	2,176,199
Amortization of Deferred Acquisition Costs	823,591	746,876	718,112
Other Operating Expenses	579,428	602,717	596,821
Net Premiums Written	4,027,143	3,752,974	3,674,666
Insurance
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	152,070	128,880	119,186
Reserve for Losses and Loss Expenses	7,164,364	5,345,655	5,291,218
Unearned Premiums	2,225,647	1,574,164	1,494,068
Net Premiums Earned	2,106,363	1,777,321	1,798,191
Net Investment Income	0	0	0
Losses And Loss Expenses	1,661,032	1,141,933	1,154,928
Amortization of Deferred Acquisition Costs	332,749	251,120	261,208
Other Operating Expenses	344,012	346,857	341,658
Net Premiums Written	2,087,734	1,807,125	1,759,359
Reinsurance
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	321,991	309,756	352,596
Reserve for Losses and Loss Expenses	5,833,189	4,352,172	4,355,067
Unearned Premiums	1,415,752	1,395,334	1,266,821
Net Premiums Earned	2,042,397	1,928,304	1,888,226
Net Investment Income	0	0	0
Losses And Loss Expenses	1,626,740	1,062,264	1,021,271
Amortization of Deferred Acquisition Costs	490,842	495,756	456,904
Other Operating Expenses	105,471	135,844	145,253
Net Premiums Written	1,939,409	1,945,849	1,915,307
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	400,805	353,335	305,336
Losses And Loss Expenses	0	0	0
Amortization of Deferred Acquisition Costs	0	0	0
Other Operating Expenses	129,945	120,016	109,910
Net Premiums Written	$ 0	$ 0	$ 0