Segment information (Gross written premiums by line of business) (Details) - USD ($) $ 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	[1]	Dec. 31, 2013
Revenue from External Customer [Line Items]
Gross premiums written	$ 309,605	$ 402,509	$ 726,168	$ 1,119,224	$ 336,643	$ 357,711	$ 653,745	$ 1,010,766	$ 2,557,506	$ 2,358,865		$ 2,388,446
Property
Revenue from External Customer [Line Items]
Gross premiums written									1,077,700	1,050,109		1,152,027
Marine
Revenue from External Customer [Line Items]
Gross premiums written									471,857	576,478		570,008
Specialty
Revenue from External Customer [Line Items]
Gross premiums written									782,463	677,026		666,411
Liability
Revenue from External Customer [Line Items]
Gross premiums written									$ 225,486	$ 55,252		$ 0

[1] The results of Western World have been included in the Company's consolidated results from the October 2, 2014 date of acquisition.