Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Gross premiums written	$ 860,289	$ 831,788	$ 2,736,794	$ 2,602,575
Net premiums written	691,381	636,117	2,162,202	2,028,129
Net premiums earned	682,049	627,409	1,958,623	1,920,337
Fee income	848	874	2,447	2,551
Losses and loss adjustment expenses	(423,984)	(359,193)	(1,349,486)	(1,150,389)
Acquisition expenses, net	(120,205)	(111,279)	(339,598)	(336,378)
Other expenses	(99,818)	(97,325)	(294,378)	(289,473)
Underwriting Income Loss - Total	38,890	60,486	(22,392)	146,648
Net investment income	82,753	90,768	257,731	274,277
Marketable Securities, Gain (Loss), Excluding Other than Temporary Impairments	30,199	68,828	96,104	178,724
Net impairment losses recognized in earnings	(2,739)	(2,075)	(7,103)	(8,091)
Equity in net income of investment funds accounted for using the equity method	(30,549)	9,708	5,097	38,410
Other income	2,432	1,840	2,734	12,346
Other expenses	(6,180)	(5,796)	(24,603)	(21,987)
Interest expense	(8,125)	(7,371)	(23,604)	(22,547)
Net foreign exchange (gains) losses	60,040	(65,157)	4,753	22,069
Income before income taxes	166,721	151,231	288,717	619,849
Income tax benefit (expense)	2,267	(3,200)	4,369	(11,373)
Net income	168,988	148,031	293,086	608,476
Preferred dividends	(6,461)	(6,461)	(19,383)	(19,383)
Net income available to common shareholders	162,527	141,570	273,703	589,093
Insurance Ratios [Abstract]
Loss ratio	62.20%	57.30%	68.90%	59.90%
Acquisition expense ratio	17.50%	17.60%	17.20%	17.40%
Other operating expense ratio	14.60%	15.50%	15.00%	15.10%
Combined ratio	94.30%	90.40%	101.10%	92.40%
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	634,280	624,490	1,903,868	1,874,419
Net premiums written	472,986	431,361	1,360,540	1,307,122
Net premiums earned	437,970	411,881	1,256,380	1,246,831
Fee income	661	864	2,141	2,491
Losses and loss adjustment expenses	(290,608)	(265,411)	(889,973)	(852,716)
Acquisition expenses, net	(76,763)	(67,309)	(204,721)	(200,099)
Other expenses	(77,078)	(77,078)	(228,580)	(229,525)
Underwriting Income Loss - Total	(5,818)	2,947	(64,753)	(33,018)
Insurance Ratios [Abstract]
Loss ratio	66.40%	64.40%	70.80%	68.40%
Acquisition expense ratio	17.40%	16.10%	16.10%	15.80%
Other operating expense ratio	17.60%	18.70%	18.20%	18.40%
Combined ratio	101.40%	99.20%	105.10%	102.60%
Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	227,837	208,770	836,616	735,942
Net premiums written	218,395	204,756	801,662	721,007
Net premiums earned	244,079	215,528	702,243	673,506
Fee income	187	10	306	60
Losses and loss adjustment expenses	(133,376)	(93,782)	(459,513)	(297,673)
Acquisition expenses, net	(43,442)	(43,970)	(134,877)	(136,279)
Other expenses	(22,740)	(20,247)	(65,798)	(59,948)
Underwriting Income Loss - Total	$ 44,708	$ 57,539	$ 42,361	$ 179,666
Insurance Ratios [Abstract]
Loss ratio	54.60%	43.50%	65.40%	44.20%
Acquisition expense ratio	17.80%	20.40%	19.20%	20.20%
Other operating expense ratio	9.30%	9.40%	9.40%	8.90%
Combined ratio	81.70%	73.30%	94.00%	73.30%