Unaudited Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	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
Quarterly Financial Information Disclosure [Abstract]
Net premiums written	$ 879,900	$ 1,079,600	$ 1,056,500	$ 1,470,400	$ 820,600	$ 987,600	$ 1,112,700	$ 1,784,200	$ 4,486,329	$ 4,705,116	$ 3,948,704
Net premiums earned	1,181,400	1,294,300	1,107,500	1,064,600	1,204,600	1,313,400	1,104,600	1,153,800	4,647,754	4,776,471	4,119,825
Net investment income	155,500	163,700	158,300	151,600	160,800	164,400	174,500	173,100	629,148	672,782	596,071
Net realized and unrealized investment (losses) gains	74,600	26,100	78,200	(112,200)	(83,200)	293,200	46,000	145,500	66,692	401,482	591,707
Other income	3,100	1,400	1,600	1,800	5,100	3,300	800	1,300	7,915	10,470	22,312
Total revenues	1,414,600	1,485,500	1,345,600	1,105,800	1,287,300	1,774,300	1,325,900	1,473,700	5,351,509	5,861,205	5,418,342
Losses and loss expenses and life policy benefits	1,069,200	881,700	814,500	1,607,200	817,800	748,900	704,600	1,012,400	4,372,570	3,283,618	2,295,296
Acquisition costs	238,800	262,500	229,200	207,900	246,600	261,600	244,100	220,100	938,361	972,537	885,214
Other operating expenses	113,000	103,800	113,700	104,300	133,200	118,200	160,200	128,100	434,846	539,751	430,808
Interest expense	12,300	12,200	12,200	12,300	12,200	12,300	12,800	7,100	48,949	44,413	28,301
Amortization of intangible assets	8,900	9,500	9,200	8,800	8,800	10,000	7,800	4,800	36,405	31,461	(6,133)
Net foreign exchange (gains) losses	(14,700)	(10,600)	(8,700)	(700)	8,300	27,100	(11,000)	(3,600)	(34,675)	20,686	1,464
Total expenses	1,427,500	1,259,100	1,170,100	1,939,800	1,226,900	1,178,100	1,118,500	1,368,900	5,796,456	4,892,466	3,634,950
(Loss) income before taxes and interest in (losses) earnings of equity investments	(12,900)	226,400	175,500	(834,000)	60,400	596,200	207,400	104,800	(444,947)	968,739	1,783,392
Income tax expense (benefit)	3,300	41,800	50,100	(26,300)	10,900	72,600	17,800	27,600	68,972	128,784	262,090
Interest in (losses) earnings of equity investments	(1,400)	(4,500)	(1,200)	700	7,500	1,300	1,300	2,400	(6,372)	12,597	15,552
Net (loss) income	(17,600)	180,100	124,200	(807,000)	57,000	524,900	190,900	79,600	(520,291)	852,552	1,536,854
Preferred dividends	15,400	14,400	8,600	8,600	8,600	8,600	8,600	8,600	47,020	34,525	34,525
Net (loss) income available to common shareholders	$ (33,000)	$ 165,700	$ 115,600	$ (815,600)	$ 48,400	$ 516,300	$ 182,300	$ 71,000	$ (567,311)	$ 818,027	$ 1,502,329
Basic net (loss) income	$ (0.49)	$ 2.45	$ 1.71	$ (11.99)	$ 0.66	$ 6.86	$ 2.36	$ 0.87	$ (8.40)	$ 10.65	$ 23.93
Diluted net (loss) income	$ (0.49)	$ 2.43	$ 1.69	$ (11.99)	$ 0.65	$ 6.76	$ 2.31	$ 0.85	$ (8.40)	$ 10.46	$ 23.51
Dividends declared per common share	$ 0.60	$ 0.60	$ 0.60	$ 0.55	$ 0.55	$ 0.50	$ 0.50	$ 0.50	$ 2.35	$ 2.05	$ 1.88