SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION (DETAILS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	$ 438,636	$ 471,782	$ 466,987
Reserve for Losses and Loss Expenses	9,697,827	9,646,285	9,596,797
Unearned Premiums	2,969,498	2,760,889	2,735,376
Net Premiums Earned	3,705,625	3,686,417	3,870,999
Net Investment Income	353,335	305,336	342,766
Losses And Loss Expenses	2,204,197	2,176,199	2,186,722
Amortization of Deferred Acquisition Costs	746,876	718,112	737,197
Other Operating Expenses	602,717	596,821	621,876
Net Premiums Written	3,752,974	3,674,666	3,906,975
Insurance [Member]
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	128,880	119,186	135,111
Reserve for Losses and Loss Expenses	5,345,655	5,291,218	5,063,147
Unearned Premiums	1,574,164	1,494,068	1,506,925
Net Premiums Earned	1,777,321	1,798,191	1,830,544
Net Investment Income	0	0	0
Losses And Loss Expenses	1,141,933	1,154,928	1,131,880
Amortization of Deferred Acquisition Costs	251,120	261,208	278,804
Other Operating Expenses	346,857	341,658	341,214
Net Premiums Written	1,807,125	1,759,359	1,779,501
Reinsurance [Member]
Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	309,756	352,596	331,876
Reserve for Losses and Loss Expenses	4,352,172	4,355,067	4,533,650
Unearned Premiums	1,395,334	1,266,821	1,228,451
Net Premiums Earned	1,928,304	1,888,226	2,040,455
Net Investment Income	0	0	0
Losses And Loss Expenses	1,062,264	1,021,271	1,054,842
Amortization of Deferred Acquisition Costs	495,756	456,904	458,393
Other Operating Expenses	135,844	145,253	144,987
Net Premiums Written	1,945,849	1,915,307	2,127,474
Corporate [Member]
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	353,335	305,336	342,766
Losses And Loss Expenses	0	0	0
Amortization of Deferred Acquisition Costs	0	0	0
Other Operating Expenses	120,016	109,910	135,675
Net Premiums Written	$ 0	$ 0	$ 0