Segments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Information by business segment
Net earned premium	$ 524,251,000	$ 506,385,000	$ 1,032,731,000	$ 1,015,972,000
Other revenue	60,601,000	61,738,000	115,829,000	133,453,000
Segment revenue	584,852,000	568,123,000	1,148,560,000	1,149,425,000
Loss and LAE	334,282,000	298,986,000	681,868,000	625,507,000
Other expense	153,665,000	149,384,000	306,908,000	301,140,000
Segment expense	487,947,000	448,370,000	988,776,000	926,647,000
Segment pretax earnings (loss)	96,905,000	119,753,000	159,784,000	222,778,000
Decrease in International segment's pretax earnings due to catastrophe losses	22,800,000		72,300,000	20,600,000
Number of operating segments			6
U.S. Property & Casualty [Member]
Information by business segment
Net earned premium	78,921,000	84,695,000	159,175,000	173,625,000
Other revenue	4,787,000	6,204,000	9,666,000	19,095,000
Segment revenue	83,708,000	90,899,000	168,841,000	192,720,000
Loss and LAE	44,944,000	50,989,000	92,428,000	102,801,000
Other expense	27,169,000	23,734,000	55,575,000	54,601,000
Segment expense	72,113,000	74,723,000	148,003,000	157,402,000
Segment pretax earnings (loss)	11,595,000	16,176,000	20,838,000	35,318,000
Professional Liability [Member]
Information by business segment
Net earned premium	102,424,000	108,001,000	203,174,000	218,153,000
Other revenue	48,000	107,000	249,000	438,000
Segment revenue	102,472,000	108,108,000	203,423,000	218,591,000
Loss and LAE	71,752,000	65,342,000	138,015,000	132,942,000
Other expense	16,928,000	18,767,000	34,032,000	38,305,000
Segment expense	88,680,000	84,109,000	172,047,000	171,247,000
Segment pretax earnings (loss)	13,792,000	23,999,000	31,376,000	47,344,000
Accident & Health [Member]
Information by business segment
Net earned premium	202,117,000	187,703,000	400,657,000	374,487,000
Other revenue	1,178,000	867,000	2,194,000	1,717,000
Segment revenue	203,295,000	188,570,000	402,851,000	376,204,000
Loss and LAE	146,747,000	135,349,000	291,605,000	273,569,000
Other expense	31,577,000	30,323,000	61,995,000	59,059,000
Segment expense	178,324,000	165,672,000	353,600,000	332,628,000
Segment pretax earnings (loss)	24,971,000	22,898,000	49,251,000	43,576,000
U.S. Surety & Credit [Member]
Information by business segment
Net earned premium	50,039,000	51,871,000	101,403,000	98,620,000
Other revenue	455,000	205,000	701,000	328,000
Segment revenue	50,494,000	52,076,000	102,104,000	98,948,000
Loss and LAE	14,648,000	15,487,000	29,687,000	27,861,000
Other expense	26,997,000	26,836,000	55,252,000	53,692,000
Segment expense	41,645,000	42,323,000	84,939,000	81,553,000
Segment pretax earnings (loss)	8,849,000	9,753,000	17,165,000	17,395,000
International [Member]
Information by business segment
Net earned premium	90,717,000	73,765,000	168,164,000	149,932,000
Other revenue	894,000	1,666,000	1,902,000	4,711,000
Segment revenue	91,611,000	75,431,000	170,066,000	154,643,000
Loss and LAE	56,221,000	28,769,000	130,393,000	84,374,000
Other expense	32,355,000	30,745,000	64,020,000	59,905,000
Segment expense	88,576,000	59,514,000	194,413,000	144,279,000
Segment pretax earnings (loss)	3,035,000	15,917,000	(24,347,000)	10,364,000
Investing [Member]
Information by business segment
Net earned premium			0
Other revenue	53,126,000	52,532,000	101,033,000	106,306,000
Segment revenue	53,126,000	52,532,000	101,033,000	106,306,000
Loss and LAE			0
Other expense			0
Segment expense			0
Segment pretax earnings (loss)	53,126,000	52,532,000	101,033,000	106,306,000
Corporate and Other [Member]
Information by business segment
Net earned premium	33,000	350,000	158,000	1,155,000
Other revenue	113,000	157,000	84,000	858,000
Segment revenue	146,000	507,000	242,000	2,013,000
Loss and LAE	(30,000)	3,050,000	(260,000)	3,960,000
Other expense	18,639,000	18,979,000	36,034,000	35,578,000
Segment expense	18,609,000	22,029,000	35,774,000	39,538,000
Segment pretax earnings (loss)	$ (18,463,000)	$ (21,522,000)	$ (35,532,000)	$ (37,525,000)