Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interim Reporting [Line Items]
Net premiums earned									$ 5,895,070	$ 6,309,521	$ 6,090,437
Underwriting profit (loss)									585,907	245,191	3,968
Net income (loss) attributable to ordinary shareholders	139,500	72,384	(279,261)	255,717	300,780	135,648	272,698	350,790
Earnings (loss) per ordinary share and ordinary share equivalent - basic (in dollars per share)	$ 0.54	$ 0.27	$ (1.03)	$ 0.93	$ 1.07	$ 0.48	$ 0.94	$ 1.19	$ 0.71	$ 3.68	$ 2.12
Earnings (loss) per ordinary share and ordinary share equivalent - diluted (in dollars per share)	$ 0.53	$ 0.27	$ (1.03)	$ 0.91	$ 1.05	$ 0.47	$ 0.93	$ 1.17	$ 0.69	$ 3.63	$ 2.10
Operating Segments [Member] | Property and casualty operations
Interim Reporting [Line Items]
Net premiums earned	1,413,161	1,453,673	1,438,076	1,412,528	1,510,727	1,550,629	1,488,497	1,464,246	5,717,438	6,014,099	5,765,978
Underwriting profit (loss)	218,851	144,375	167,946	144,874	101,450	76,900	92,124	180,588	676,046	451,062	216,132
Corporate and other
Interim Reporting [Line Items]
Net premiums earned	$ 23,064	$ 19,739	$ 58,518	$ 76,311	$ 81,398	$ 74,683	$ 73,896	$ 65,445