Segment Information - Net premiums written by geographical location (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 7,951,063	$ 5,944,841	$ 6,199,404
Life operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	62,239	177,632	295,422
Life operations | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	62,276	91,979	117,948
Life operations | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	(37)	85,653	177,474
Operating Segments [Member] | Total P And C Segment [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	7,888,824	5,767,209	5,903,982
Operating Segments [Member] | Total P And C Segment [Member] | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	781,618	636,109	658,041
Operating Segments [Member] | Total P And C Segment [Member] | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	3,045,031	2,528,196	2,650,916
Operating Segments [Member] | Total P And C Segment [Member] | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	3,628,258	2,124,117	2,111,065
Operating Segments [Member] | Total P And C Segment [Member] | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 433,917	$ 478,787	$ 483,960