Consolidated Statements of Operations and Comprehensive Income (Loss) (USD $) In Thousands, except Share data, 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
Revenues
Gross premiums written									$ 4,718,235		$ 4,633,054		$ 4,885,266
Net premiums written	920,300	1,043,200	1,136,000	1,473,300	879,900	1,079,600	1,056,500	1,470,400	4,572,860		4,486,329		4,705,116
(Increase) decrease in unearned premiums									(86,921)		161,425		71,355
Net premiums earned	1,168,100	1,237,100	1,090,900	989,800	1,181,400	1,294,300	1,107,500	1,064,600	4,485,939		4,647,754		4,776,471
Net investment income	135,700	135,300	153,500	146,900	155,500	163,700	158,300	151,600	571,338		629,148		672,782
Net realized and unrealized investment gains	5,100	257,400	38,100	192,700	74,600	26,100	78,200	(112,200)	493,409		66,692		401,482
Other income	3,800	2,700	2,600	2,800	3,100	1,400	1,600	1,800	11,920		7,915		10,470
Total revenues	1,312,700	1,632,500	1,285,100	1,332,200	1,414,600	1,485,500	1,345,600	1,105,800	5,562,606		5,351,509		5,861,205
Expenses
Losses and loss expenses and life policy benefits	800,900	721,100	706,100	576,500	1,069,200	881,700	814,500	1,607,200	2,804,610		4,372,570		3,283,618
Acquisition costs	245,500	247,100	232,700	211,600	238,800	262,500	229,200	207,900	936,909		938,361		972,537
Other operating expenses	112,300	94,700	106,200	98,200	113,000	103,800	113,700	104,300	411,374		434,846		539,751
Interest expense	12,200	12,200	12,200	12,200	12,300	12,200	12,200	12,300	48,895		48,949		44,413
Amortization of intangible assets	5,100	8,900	8,900	8,900	8,900	9,500	9,200	8,800	31,799		36,405		31,461
Net foreign exchange losses (gains)	3,400	2,000	(7,700)	2,600	(14,700)	(10,600)	(8,700)	(700)	175		(34,675)		20,686
Total expenses	1,179,400	1,086,000	1,058,400	910,000	1,427,500	1,259,100	1,170,100	1,939,800	4,233,762		5,796,456		4,892,466
Income (loss) before taxes and interest in earnings (losses) of equity investments	133,300	546,500	226,700	422,200	(12,900)	226,400	175,500	(834,000)	1,328,844		(444,947)		968,739
Income tax expense	22,800	64,100	50,100	67,200	3,300	41,800	50,100	(26,300)	204,284		68,972		128,784
Interest in earnings (losses) of equity investments	1,000	4,300	(500)	5,100	(1,400)	(4,500)	(1,200)	700	9,954		(6,372)		12,597
Net income (loss)	111,500	486,700	176,100	360,100	(17,600)	180,100	124,200	(807,000)	1,134,514		(520,291)		852,552
Preferred dividends	15,400	15,400	15,400	15,400	15,400	14,400	8,600	8,600	61,622		47,020		34,525
Net income (loss) available to common shareholders	96,100	471,300	160,700	344,700	(33,000)	165,700	115,600	(815,600)	1,072,892		(567,311)		818,027
Comprehensive income (loss)
Net income (loss)	111,500	486,700	176,100	360,100	(17,600)	180,100	124,200	(807,000)	1,134,514		(520,291)		852,552
Change in currency translation adjustment									28,488		(11,834)		(66,742)
Change in pension obligation									(4,294)		(3,917)		(9,221)
Change in unrealized lossess on investments									(953)		(949)		(4,908)
Other Comprehensive Income Loss, total									23,241		(16,700)		(80,871)
Comprehensive income (loss)									$ 1,157,755		$ (536,991)		$ 771,681
Per share data
Basic net income (loss)	$ 1.58	$ 7.62	$ 2.52	$ 5.27	$ (0.49)	$ 2.45	$ 1.71	$ (11.99)	$ 17.05		$ (8.40)		$ 10.65
Diluted net income (loss)	$ 1.56	$ 7.53	$ 2.50	$ 5.24	$ (0.49)	$ 2.43	$ 1.69	$ (11.99)	$ 16.87	[1]	$ (8.40)	[1]	$ 10.46	[1]
Weighted average number of common shares outstanding									62,915,992		67,558,732		76,839,519
Weighted average number of common shares and common share equivalents outstanding									63,615,748		67,558,732		78,234,312
Dividends declared per common share	$ 0.62	$ 0.62	$ 0.62	$ 0.62	$ 0.60	$ 0.60	$ 0.60	$ 0.55	$ 2.48		$ 2.35		$ 2.05

[1] At December 31, 2012, 2011 and 2010, share based awards to purchase 554.7 thousand, 2,854.4 thousand and 489.7 thousand common shares, respectively, were excluded from the calculation of diluted weighted average number of common shares and common share equivalents outstanding because their exercise prices were greater than the average market price of the common shares. In addition, dilutive securities, in the form of share options and other, of 687.3 thousand shares were not included in the weighted average number of common shares and common share equivalents outstanding for the purpose of computing the diluted net loss per share because to do so would have been anti-dilutive for year ended December 31, 2011.