Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Information by business segment
Net earned premium	$ 550,183	$ 544,256	$ 524,251	$ 508,480	$ 509,786	$ 516,166	$ 506,385	$ 509,587	$ 2,127,170	$ 2,041,924	$ 2,037,235
Other revenue	64,738	66,268	60,601	55,228	67,095	59,782	61,738	71,715	246,835	260,330	281,281
Total revenue	614,921	610,524	584,852	563,708	576,881	575,948	568,123	581,302	2,374,005	2,302,254	2,318,516
Loss and LAE	337,007	380,372	334,282	347,586	290,384	297,138	298,986	326,521	1,399,247	1,213,029	1,215,759
Other expense	165,484	147,360	153,665	153,243	151,460	146,798	149,384	151,756	619,752	599,398	584,206
Segment Expense									2,018,999	1,812,427	1,799,965
Segment Pre Tax Earnings Loss									355,006	489,827	518,551
U.S. Property & Casualty
Information by business segment
Net earned premium									333,410	339,513	379,439
Other revenue									23,951	31,201	53,105
Total revenue									357,361	370,714	432,544
Loss and LAE									201,017	191,108	201,311
Other expense									110,184	103,229	115,198
Segment Expense									311,201	294,337	316,509
Segment Pre Tax Earnings Loss									46,160	76,377	116,035
Professional Liability [Member]
Information by business segment
Net earned premium									410,816	425,226	444,534
Other revenue									912	981	(212)
Total revenue									411,728	426,207	444,322
Loss and LAE									328,503	265,465	276,558
Other expense									59,036	74,524	59,744
Segment Expense									387,539	339,989	336,302
Segment Pre Tax Earnings Loss									24,189	86,218	108,020
Accident & Health
Information by business segment
Net earned premium									803,462	760,034	741,539
Other revenue									4,687	3,875	5,180
Total revenue									808,149	763,909	746,719
Loss and LAE									586,210	556,848	540,917
Other expense									121,261	117,308	117,189
Segment Expense									707,471	674,156	658,106
Segment Pre Tax Earnings Loss									100,678	89,753	88,613
U.S. Surety & Credit
Information by business segment
Net earned premium									210,535	199,908	182,627
Other revenue									1,247	580	274
Total revenue									211,782	200,488	182,901
Loss and LAE									52,206	52,940	54,618
Other expense									113,932	109,685	98,518
Segment Expense									166,138	162,625	153,136
Segment Pre Tax Earnings Loss									45,644	37,863	29,765
International [Member]
Information by business segment
Net earned premium									368,748	316,186	256,122
Other revenue									5,309	7,344	23,518
Total revenue									374,057	323,530	279,640
Loss and LAE									233,879	143,412	94,550
Other expense									136,750	120,956	115,342
Segment Expense									370,629	264,368	209,892
Segment Pre Tax Earnings Loss									3,428	59,162	69,748
Investing [Member]
Information by business segment
Net earned premium									0	0	0
Other revenue									211,245	215,498	198,612
Total revenue									211,245	215,498	198,612
Loss and LAE									0	0	0
Other expense									0	0	0
Segment Expense									0	0	0
Segment Pre Tax Earnings Loss									211,245	215,498	198,612
Corporate and Other [Member]
Information by business segment
Net earned premium									199	1,057	32,974
Other revenue									(516)	851	804
Total revenue									(317)	1,908	33,778
Loss and LAE									(2,568)	3,256	47,805
Other expense									78,589	73,696	78,215
Segment Expense									76,021	76,952	126,020
Segment Pre Tax Earnings Loss									$ (76,338)	$ (75,044)	$ (92,242)