Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interim Reporting [Line Items]
Net premiums earned									$ 8,226,425	$ 5,895,070	$ 6,309,521
Underwriting profit (loss)									$ 587,409	$ 585,907	$ 245,191
Net income (loss) attributable to ordinary shareholders	$ 228,550	$ 27,282	$ 915,039	$ 36,281	$ 139,499	$ 72,384	$ (279,260)	$ 255,717
Earnings (loss) per ordinary share and ordinary share equivalent - basic (in dollars per share)	$ 0.77	$ 0.09	$ 3.16	$ 0.14	$ 0.54	$ 0.27	$ (1.03)	$ 0.93	$ 4.22	$ 0.71	$ 3.68
Earnings (loss) per ordinary share and ordinary share equivalent - diluted (in dollars per share)	$ 0.76	$ 0.09	$ 3.11	$ 0.14	$ 0.53	$ 0.27	$ (1.03)	$ 0.91	$ 4.15	$ 0.69	$ 3.63
Operating Segments [Member] | Property and casualty operations
Interim Reporting [Line Items]
Net premiums earned	$ 2,375,155	$ 2,405,740	$ 2,063,795	$ 1,319,494	$ 1,413,161	$ 1,453,673	$ 1,438,076	$ 1,412,528	$ 8,164,184	$ 5,717,438	$ 6,014,099
Underwriting profit (loss)	183,458	114,118	208,779	146,836	218,850	144,375	167,947	144,874	$ 653,191	$ 676,046	$ 451,062
Corporate and other
Interim Reporting [Line Items]
Net premiums earned	$ 11,665	$ 17,812	$ 18,258	$ 14,506	$ 23,064	$ 19,739	$ 58,518	$ 76,311