Reserve for losses and loss expenses (Prior year development) (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015		Jun. 30, 2016	Jun. 30, 2015
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years	[1]	$ (62,781)	$ (70,718)		$ (116,520)	$ (154,277)
Validus Re
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(30,877)	(30,879)		(56,561)	(55,575)
Talbot
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(28,328)	(35,586)		(51,048)	(87,273)
Western World
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(3,158)	(4,253)	[1]	(7,584)	(10,585)	[1]
AlphaCat
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(418)			(1,327)	(844)
Property
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(21,440)	(33,477)	[1]	(63,340)	(72,697)	[1]
Property | Validus Re
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(9,468)	(15,928)		(32,300)	(30,824)
Property | Talbot
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(10,094)	(16,683)		(28,540)	(37,435)
Property | Western World
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(1,582)	(866)	[1]	(2,023)	(3,594)	[1]
Property | AlphaCat
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(296)			(477)	(844)
Marine
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(18,946)	(24,909)	[1]	(12,427)	(51,993)	[1]
Marine | Validus Re
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(10,018)	(11,099)		(6,463)	(15,669)
Marine | Talbot
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(8,928)	(13,810)		(5,964)	(36,324)
Specialty
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(20,819)	(8,945)	[1]	(35,192)	(22,596)	[1]
Specialty | Validus Re
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(11,391)	(3,852)		(17,798)	(9,082)
Specialty | Talbot
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(9,306)	(5,093)		(16,544)	(13,514)
Specialty | AlphaCat
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(122)			(850)	0
Liability
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		(1,576)	(3,387)	[1]	(5,561)	(6,991)	[1]
Liability | Western World
Increase (decrease) in net reserves for losses and loss expenses in respect of losses occurring in:
Net favorable development on prior years		$ (1,576)	$ (3,387)	[1]	$ (5,561)	$ (6,991)	[1]

[1] Upon closing the acquisition of Western World, an adjustment of $15,586 was made to increase net reserves to reflect fair value. This adjustment was amortized to income through a reduction in losses and loss expenses of $2,892 and $6,115, respectively, during the three and six months ended June 30, 2015, benefiting the loss ratio by 4.4 and 4.6 percentage points, respectively. The remaining fair value adjustment of $4,864 was fully amortized during 2015.