Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interim Reporting [Line Items]
Net premiums earned										$ 9,777,934	$ 8,226,425	$ 5,895,070
Underwriting profit (loss)										$ 547,325	$ 587,409	$ 585,907
Net income (loss) attributable to ordinary shareholders		$ 304,700	$ 70,601	$ 43,782	$ 21,885	$ 228,550	$ 27,282	$ 915,039	$ 36,281
Earnings (loss) per ordinary share and ordinary share equivalent - basic (in dollars per share)		$ 1.13	$ 0.26	$ 0.16	$ 0.07	$ 0.77	$ 0.09	$ 3.16	$ 0.14	$ 1.58	$ 4.22	$ 0.71
Earnings (loss) per ordinary share and ordinary share equivalent - diluted (in dollars per share)		$ 1.12	$ 0.25	$ 0.15	$ 0.07	$ 0.76	$ 0.09	$ 3.11	$ 0.14	$ 1.56	$ 4.15	$ 0.69
Corporate and other
Interim Reporting [Line Items]
Net premiums earned	[1]									$ 12,047	$ 62,241	$ 177,632
Underwriting profit (loss)	[1]									(23,328)	(65,782)	(90,139)
Operating Segments [Member] | Property and casualty operations
Interim Reporting [Line Items]
Net premiums earned		$ 2,454,060	$ 2,431,677	$ 2,528,704	$ 2,351,446	$ 2,375,155	$ 2,405,740	$ 2,063,795	$ 1,319,494	9,765,887	8,164,184	5,717,438
Underwriting profit (loss)		126,420	166,851	101,893	175,489	183,458	114,118	208,779	146,836	$ 570,653	$ 653,191	$ 676,046
Operating Segments [Member] | Corporate and other
Interim Reporting [Line Items]
Net premiums earned		$ 3,015	$ 2,360	$ 3,508	$ 3,164	$ 11,665	$ 17,812	$ 18,258	$ 14,506

[1] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes, as well as the Company's Run-Off Life Operations.