Variable interest entities (Assets and Liabilities of Consolidated VIEs) (Details) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016 USD ($) subsidiaries	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) subsidiaries	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Variable Interest Entity [Line Items]
(Income) attributable to AlphaCat investors		$ (7,080)	$ (5,564)	$ (6,114)	$ (4,600)	$ (974)	$ (1,438)	$ 0	$ 0	$ (23,358)	$ (2,412)	$ 0
Accounts payable and accrued expenses
Variable Interest Entity [Line Items]
(Income) attributable to AlphaCat investors										$ (17,068)	(2,412)
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Variable Funding Notes, Typical Minimum Duration										12 months
AlphaCat Sidecars | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets		40,041				206,581				$ 40,041	206,581
Total liabilities		3,206				14,804				3,206	14,804
AlphaCat ILS Funds - Lower Risk (a) | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets	[1]	1,498,276				1,268,070				1,498,276	1,268,070
Total liabilities	[1]	42,457				143,371				42,457	143,371
AlphaCat ILS Funds - Higher Risk (a) | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets	[1]	972,633				522,867				972,633	522,867
Total liabilities	[1]	381,332				300,122				381,332	300,122
AlphaCat Re & Master Fund | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total assets		2,510,415				1,615,779				2,510,415	1,615,779
Total liabilities		$ 2,510,245				1,615,609				$ 2,510,245	1,615,609
BetaCat ILS Funds | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Number of Consolidated Funds | subsidiaries		1								1
Total assets		$ 82,471				64,221				$ 82,471	64,221
Total liabilities		$ 30,663				$ 2,472				$ 30,663	$ 2,472

[1] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOmNlNDcwMTY5ZWM5MzQxZjk5YzIzNDFhMDczZjllOTQ1fFRleHRTZWxlY3Rpb246NTg2NUZFREY5MjRCQjBDODYwQzgwOEMwNzJCQTM1MkYM}