Segment Information - Net premiums written by geographical location (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 5,944,841	$ 6,199,404	$ 6,281,451
Life operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	177,632	295,422	324,432
Life operations | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	91,979	117,948	129,997
Life operations | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	85,653	177,474	194,435
Operating Segments [Member] | Total P And C Segment [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	5,767,209	5,903,982	5,957,019
Operating Segments [Member] | Total P And C Segment [Member] | Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	636,109	658,041	644,566
Operating Segments [Member] | Total P And C Segment [Member] | United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	2,528,196	2,650,916	2,626,855
Operating Segments [Member] | Total P And C Segment [Member] | Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	2,124,117	2,111,065	2,202,209
Operating Segments [Member] | Total P And C Segment [Member] | Other
Revenues from External Customers and Long-Lived Assets [Line Items]
Net premiums written	$ 478,787	$ 483,960	$ 483,389