Segment information (Gross written premiums by line of business) (Details) - USD ($) $ 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
Revenue from External Customer [Line Items]
Gross premiums written	$ 339,454	$ 372,418	$ 764,042	$ 1,172,791	$ 309,605	$ 402,509	$ 726,168	$ 1,119,224	$ 2,648,705	$ 2,557,506	$ 2,358,865
Property
Revenue from External Customer [Line Items]
Gross premiums written									1,128,523	1,077,700	1,050,109	[1]
Marine
Revenue from External Customer [Line Items]
Gross premiums written									363,656	471,857	576,478	[1]
Specialty
Revenue from External Customer [Line Items]
Gross premiums written									927,746	782,463	677,026	[1]
Liability
Revenue from External Customer [Line Items]
Gross premiums written									$ 228,780	$ 225,486	$ 55,252	[1]

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