Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per 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
Interim Reporting [Line Items]
Net premiums earned									$ 6,090,441	$ 5,690,130	$ 5,414,061
Underwriting profit (loss)	(90,677)	114,203	129,372	63,240	(114,138)	(22,200)	67,049	(328,064)	3,970	(619,038)	72,011
Net income (loss) attributable to ordinary shareholders	81,448	171,902	221,156	176,628	(515,537)	42,398	225,663	(227,284)	651,134	(474,760)	585,472
Net income (loss) per ordinary share and ordinary share equivalent - basic (in Dollars per share)	$ 0.27	$ 0.57	$ 0.71	$ 0.56	$ (1.62)	$ 0.14	$ 0.73	$ (0.73)	$ 2.12	$ (1.52)	$ 1.74
Net income (loss) per ordinary share and ordinary share equivalent - diluted (in Dollars per share)	$ 0.27	$ 0.56	$ 0.71	$ 0.56	$ (1.62)	$ 0.14	$ 0.69	$ (0.73)	$ 2.10	$ (1.52)	$ 1.73
Property and casualty operations
Interim Reporting [Line Items]
Net premiums earned	1,538,559	1,468,553	1,400,972	1,357,898	1,386,759	1,362,532	1,306,125	1,271,696	5,765,982	5,327,112	5,031,137
Underwriting profit (loss)									216,138	(397,353)	262,494
Life Insurance Segment [Member]
Interim Reporting [Line Items]
Net premiums earned	80,829	79,487	85,623	78,520	90,323	90,794	92,214	89,687	324,459	363,018	382,924
Underwriting profit (loss)									$ (212,168)	$ (221,685)	$ (190,483)