Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Information by business segment
Net earned premium	$ 566,503	$ 563,650	$ 565,331	$ 547,141	$ 550,183	$ 544,256	$ 524,251	$ 508,480	$ 2,242,625	$ 2,127,170	$ 2,041,924
Other revenue	85,840	68,023	66,957	62,382	64,738	66,268	60,601	55,228	283,202	246,835	260,330
Total revenue	652,343	631,673	632,288	609,523	614,921	610,524	584,852	563,708	2,525,827	2,374,005	2,302,254
Loss and LAE	335,744	304,014	336,825	328,928	337,007	380,372	334,282	347,586	1,305,511	1,399,247	1,213,029
Other expense	167,070	174,040	161,144	163,635	165,484	147,360	153,665	153,243	665,889	619,752	599,398
Segment Expense									1,971,400	2,018,999	1,812,427
Segment Pre Tax Earnings Loss									554,427	355,006	489,827
United States Property And Casualty [Member]
Information by business segment
Net earned premium									354,050	333,410	339,513
Other revenue									18,865	23,951	31,201
Total revenue									372,915	357,361	370,714
Loss and LAE									209,286	201,017	191,108
Other expense									116,398	110,184	103,229
Segment Expense									325,684	311,201	294,337
Segment Pre Tax Earnings Loss									47,231	46,160	76,377
Professional Liability [Member]
Information by business segment
Net earned premium									394,687	410,816	425,226
Other revenue									731	912	981
Total revenue									395,418	411,728	426,207
Loss and LAE									229,873	328,503	265,465
Other expense									66,721	59,036	74,524
Segment Expense									296,594	387,539	339,989
Segment Pre Tax Earnings Loss									98,824	24,189	86,218
Accident And Health [Member]
Information by business segment
Net earned premium									831,827	758,270	705,408
Other revenue									4,918	4,684	3,872
Total revenue									836,745	762,954	709,280
Loss and LAE									601,076	552,292	506,994
Other expense									122,232	116,336	110,942
Segment Expense									723,308	668,628	617,936
Segment Pre Tax Earnings Loss									113,437	94,326	91,344
United States Surety And Credit [Member]
Information by business segment
Net earned premium									207,955	210,535	199,908
Other revenue									843	1,247	580
Total revenue									208,798	211,782	200,488
Loss and LAE									38,535	52,206	52,940
Other expense									113,619	113,932	109,685
Segment Expense									152,154	166,138	162,625
Segment Pre Tax Earnings Loss									56,644	45,644	37,863
International [Member]
Information by business segment
Net earned premium									412,853	368,748	316,186
Other revenue									5,005	5,309	7,344
Total revenue									417,858	374,057	323,530
Loss and LAE									189,410	233,879	143,412
Other expense									146,807	136,750	120,956
Segment Expense									336,217	370,629	264,368
Segment Pre Tax Earnings Loss									81,641	3,428	59,162
Investing [Member]
Information by business segment
Net earned premium									0	0	0
Other revenue									252,754	211,245	215,498
Total revenue									252,754	211,245	215,498
Loss and LAE									0	0	0
Other expense									0	0	0
Segment Expense									0	0	0
Segment Pre Tax Earnings Loss									252,754	211,245	215,498
Corporate & Other [Member]
Information by business segment
Net earned premium									41,253	45,391	55,683
Other revenue									86	(513)	854
Total revenue									41,339	44,878	56,537
Loss and LAE									37,331	31,350	53,110
Other expense									100,112	83,514	80,062
Segment Expense									137,443	114,864	133,172
Segment Pre Tax Earnings Loss									$ (96,104)	$ (69,986)	$ (76,635)