Reserve for losses and loss expenses (Prior year development) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years	[1]	$ (222,533)	$ (216,192)	$ (306,137)
Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(222,533)	(216,192)	(306,137)
Operating Segments | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(70,918)	(100,815)	(136,978)
Operating Segments | Specialty - short tail
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(108,227)	(65,517)	(116,204)
Operating Segments | Specialty - other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(43,388)	(49,860)	(52,955)
Operating Segments | Reinsurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(113,302)	(135,626)	(137,395)
Operating Segments | Reinsurance Segment | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(34,063)	(61,102)	(71,150)
Operating Segments | Reinsurance Segment | Specialty - short tail
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(76,620)	(70,439)	(65,193)
Operating Segments | Reinsurance Segment | Specialty - other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(2,619)	(4,085)	(1,052)
Operating Segments | Insurance Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(96,692)	(77,829)	(160,545)
Operating Segments | Insurance Segment | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(26,202)	(37,855)	(57,631)
Operating Segments | Insurance Segment | Specialty - short tail
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(29,721)	5,801	(51,011)
Operating Segments | Insurance Segment | Specialty - other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(40,769)	(45,775)	(51,903)
Operating Segments | Asset Management Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(12,539)	(2,737)	(8,197)
Operating Segments | Asset Management Segment | Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(10,653)	(1,858)	(8,197)
Operating Segments | Asset Management Segment | Specialty - short tail
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		(1,886)	(879)	0
Operating Segments | Asset Management Segment | Specialty - other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net favorable development on prior years		$ 0	$ 0	$ 0

[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 fully amortized to income through a reduction in losses and loss expenses of $10,979 and $4,607 during the years ended December 31, 2015 and 2014, respectively.