Segment Information (Details) (USD $)	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
Segment Reporting Information [Line Items]
Gross premiums written									$ 3,436,456,000	[1]	$ 3,266,787,000	[1]	$ 3,592,931,000	[1]
Net premiums written	511,124,000	691,381,000	706,543,000	764,278,000	482,911,000	636,117,000	624,258,000	767,754,000	2,673,326,000		2,511,040,000		2,763,112,000
Net premiums earned	673,192,000	682,049,000	642,879,000	633,695,000	632,146,000	627,409,000	623,011,000	669,917,000	2,631,815,000		2,552,483,000		2,842,745,000
Fee income	982,000	848,000	784,000	815,000	2,814,000	874,000	883,000	794,000	3,429,000		5,365,000		3,462,000
Losses and loss adjustment expenses	(378,067,000)	(423,984,000)	(431,622,000)	(493,880,000)	(367,326,000)	(359,193,000)	(363,145,000)	(428,051,000)	(1,727,553,000)		(1,517,715,000)		(1,654,674,000)
Acquisition expenses, net	(123,339,000)	(120,205,000)	(110,639,000)	(108,754,000)	(104,824,000)	(111,279,000)	(107,475,000)	(117,624,000)	(462,937,000)		(441,202,000)		(493,560,000)
Other expenses									(400,100,000)		(403,927,000)		(361,907,000)
Underwriting income									44,654,000		195,004,000		336,066,000
Net investment income	80,467,000	82,753,000	86,671,000	88,307,000	90,601,000	90,768,000	90,537,000	92,972,000	338,198,000		364,878,000		390,131,000
Net realized gains (losses)	14,542,000	30,199,000	45,210,000	20,695,000	74,027,000	68,828,000	62,114,000	47,782,000	110,646,000		252,751,000		143,582,000
Net impairment losses recognized in earnings	(1,959,000)	(2,739,000)	(1,684,000)	(2,680,000)	(3,230,000)	(2,075,000)	(4,410,000)	(1,606,000)	(9,062,000)		(11,321,000)		(66,133,000)
Equity in net income of investment funds accounted for using the equity method	(14,702,000)	(30,549,000)	5,973,000	29,673,000	22,990,000	9,708,000	(348,000)	29,050,000	(9,605,000)		61,400,000		167,819,000
Other income	(4,848,000)	2,432,000	(4,265,000)	4,567,000	6,165,000	1,840,000	4,528,000	5,978,000	(2,114,000)		18,511,000		20,016,000
Other expenses									(31,380,000)		(28,868,000)		(30,231,000)
Interest expense	(8,087,000)	(8,125,000)	(7,758,000)	(7,721,000)	(7,460,000)	(7,371,000)	(7,916,000)	(7,260,000)	(31,691,000)		(30,007,000)		(24,440,000)
Net foreign exchange (gains) losses	12,613,000	60,040,000	(18,375,000)	(36,912,000)	6,039,000	(65,157,000)	48,625,000	38,601,000	17,366,000		28,108,000		(39,157,000)
Income before income taxes									427,012,000		850,456,000		897,653,000
Income tax benefit (expense)	4,977,000	2,267,000	1,731,000	371,000	3,505,000	(3,200,000)	(1,420,000)	(6,753,000)	9,346,000		(7,868,000)		(20,708,000)
Net income	143,272,000	168,988,000	98,342,000	25,756,000	234,112,000	148,031,000	243,451,000	216,994,000	436,358,000		842,588,000		876,945,000
Dividends declared on preferred shares	(6,461,000)	(6,461,000)	(6,461,000)	(6,461,000)	(6,461,000)	(6,461,000)	(6,461,000)	(6,461,000)	(25,844,000)		(25,844,000)		(25,844,000)
Net income available to common shareholders	136,811,000	162,527,000	91,881,000	19,295,000	227,651,000	141,570,000	236,990,000	210,533,000	410,514,000		816,744,000		851,101,000
Insurance Ratios [Abstract]
Loss ratio									65.60%		59.50%		58.20%
Acquisition expense ratio									17.50%	[2]	17.20%	[2]	17.20%	[2]
Other operating expense ratio									15.20%		15.80%		12.70%
Combined ratio									98.30%		92.50%		88.10%
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									2,444,485,000	[1]	2,402,202,000	[1]	2,512,127,000	[1]
Net premiums written									1,721,279,000	[1]	1,658,963,000	[1]	1,704,284,000	[1]
Net premiums earned									1,679,047,000	[1]	1,651,106,000	[1]	1,688,519,000	[1]
Fee income									2,870,000		3,252,000		3,362,000
Losses and loss adjustment expenses									(1,172,742,000)		(1,117,564,000)		(1,139,415,000)
Acquisition expenses, net									(278,696,000)		(263,201,000)		(238,261,000)
Other expenses									(306,801,000)		(312,404,000)		(281,340,000)
Underwriting income									(76,322,000)		(38,811,000)		32,865,000
Insurance Ratios [Abstract]
Loss ratio									69.80%		67.70%		67.50%
Acquisition expense ratio									16.40%	[2]	15.70%	[2]	13.90%	[2]
Other operating expense ratio									18.30%		18.90%		16.70%
Combined ratio									104.50%		102.30%		98.10%
Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									998,520,000	[1]	874,957,000	[1]	1,093,940,000	[1]
Net premiums written									952,047,000	[1]	852,077,000	[1]	1,058,828,000	[1]
Net premiums earned									952,768,000	[1]	901,377,000	[1]	1,154,226,000	[1]
Fee income									559,000		2,113,000		100,000
Losses and loss adjustment expenses									(554,811,000)		(400,151,000)		(515,259,000)
Acquisition expenses, net									(184,241,000)		(178,001,000)		(255,299,000)
Other expenses									(93,299,000)		(91,523,000)		(80,567,000)
Underwriting income									$ 120,976,000		$ 233,815,000		$ 303,201,000
Insurance Ratios [Abstract]
Loss ratio									58.20%		44.40%		44.60%
Acquisition expense ratio									19.30%	[2]	19.70%	[2]	22.10%	[2]
Other operating expense ratio									9.80%		10.20%		7.00%
Combined ratio									87.30%		74.30%		73.70%

[1] Certain amounts included in the gross premiums written of each segment are related to intersegment transactions. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

[2] The acquisition expense ratio is adjusted to include policy-related fee income.