Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Information by business segment
Net earned premium	$ 572,248	$ 561,356	$ 1,134,860	$ 1,122,542
Other revenue	71,326	68,075	157,644	141,255
Total revenue	643,574	629,431	1,292,504	1,263,797
Loss and LAE	327,094	339,474	648,938	672,171
Other expense	178,836	165,126	350,950	315,399
Segment expense	505,930	504,600	999,888	987,570
Segment pretax earnings (loss)	137,644	124,831	292,616	276,227
U.S. Property & Casualty [Member]
Information by business segment
Net earned premium	88,536	91,432	185,588	184,963
Other revenue	4,070	4,245	9,946	11,429
Total revenue	92,606	95,677	195,534	196,392
Loss and LAE	46,733	55,238	93,388	107,394
Other expense	29,055	25,452	58,148	52,757
Segment expense	75,788	80,690	151,536	160,151
Segment pretax earnings (loss)	16,818	14,987	43,998	36,241
Professional Liability [Member]
Information by business segment
Net earned premium	88,127	92,444	173,577	185,223
Other revenue	195	739	470	325
Total revenue	88,322	93,183	174,047	185,548
Loss and LAE	52,189	55,435	103,321	111,821
Other expense	17,420	18,124	34,545	35,872
Segment expense	69,609	73,559	137,866	147,693
Segment pretax earnings (loss)	18,713	19,624	36,181	37,855
Accident & Health [Member]
Information by business segment
Net earned premium	240,338	217,822	472,481	434,947
Other revenue	3,862	1,140	5,502	2,330
Total revenue	244,200	218,962	477,983	437,277
Loss and LAE	172,296	160,139	341,295	320,566
Other expense	37,875	32,583	74,254	63,709
Segment expense	210,171	192,722	415,549	384,275
Segment pretax earnings (loss)	34,029	26,240	62,434	53,002
U.S. Surety & Credit [Member]
Information by business segment
Net earned premium	50,191	50,054	97,134	97,231
Other revenue	580	384	868	621
Total revenue	50,771	50,438	98,002	97,852
Loss and LAE	14,862	5,637	28,558	18,851
Other expense	27,463	27,402	53,407	53,681
Segment expense	42,325	33,039	81,965	72,532
Segment pretax earnings (loss)	8,446	17,399	16,037	25,320
International [Member]
Information by business segment
Net earned premium	104,727	105,270	205,460	210,412
Other revenue	931	1,124	1,899	1,902
Total revenue	105,658	106,394	207,359	212,314
Loss and LAE	40,765	58,997	82,380	104,916
Other expense	41,302	36,212	79,979	71,921
Segment expense	82,067	95,209	162,359	176,837
Segment pretax earnings (loss)	23,591	11,185	45,000	35,477
Investing [Member]
Information by business segment
Net earned premium	0	0	0	0
Other revenue	61,343	60,291	138,395	124,626
Total revenue	61,343	60,291	138,395	124,626
Loss and LAE	0	0	0	0
Other expense	0	0	0	0
Segment expense	0	0	0	0
Segment pretax earnings (loss)	61,343	60,291	138,395	124,626
Corporate & Other [Member]
Information by business segment
Net earned premium	329	4,334	620	9,766
Other revenue	345	152	564	22
Total revenue	674	4,486	1,184	9,788
Loss and LAE	249	4,028	(4)	8,623
Other expense	25,721	25,353	50,617	37,459
Segment expense	25,970	29,381	50,613	46,082
Segment pretax earnings (loss)	$ (25,296)	$ (24,895)	$ (49,429)	$ (36,294)