Variable interest entities (Assets and liabilities of consolidated VIEs) (Details) - Variable interest entities, primary beneficiary - USD ($) $ in Thousands		Jun. 30, 2016	Dec. 31, 2015
AlphaCat sidecars
Variable Interest Entity [Line Items]
Total assets		$ 47,693	$ 206,581
Total liabilities		10,468	14,804
AlphaCat ILS funds - Lower Risk (a)
Variable Interest Entity [Line Items]
Total assets	[1]	1,544,346	1,268,070
Total liabilities	[1]	129,842	143,371
AlphaCat ILS funds - Higher Risk (a)
Variable Interest Entity [Line Items]
Total assets	[1]	701,348	522,867
Total liabilities	[1]	147,466	300,122
AlphaCat Re and AlphaCat Master Fund
Variable Interest Entity [Line Items]
Total assets		2,431,907	1,615,779
Total liabilities		2,431,737	1,615,609
BetaCat ILS funds
Variable Interest Entity [Line Items]
Total assets		63,947	64,221
Total liabilities		$ 493	$ 2,472

[1] Lower risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of less than 7%, whereas higher risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of greater than 7%. Expected loss represents the average annual loss over the set of simulation scenarios divided by the total limit.