Segment reporting - Customer Concentration (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenue, Major Customer [Line Items]
Gross premiums written	$ 184,342	$ 145,508	$ 397,676	$ 233,095
Gross Premiums Written
Revenue, Major Customer [Line Items]
Gross premiums written	$ 184,342	$ 145,508	$ 397,676	$ 233,095
Concentration risk, percentage	100.00%	100.00%	100.00%	100.00%
Gross Premiums Written | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Gross premiums written			$ 397,676	$ 233,095
Concentration risk, percentage			100.00%	100.00%
Gross Premiums Written | Customer Concentration Risk | JLT Re
Revenue, Major Customer [Line Items]
Gross premiums written			$ 127,237	$ 0
Concentration risk, percentage			32.00%	0.00%
Gross Premiums Written | Customer Concentration Risk | Aon Benfield - a division of Aon plc
Revenue, Major Customer [Line Items]
Gross premiums written			$ 90,299	$ 16,793
Concentration risk, percentage			22.70%	7.20%
Gross Premiums Written | Customer Concentration Risk | Advocate Reinsurance Partners, LLC
Revenue, Major Customer [Line Items]
Gross premiums written			$ 25,916	$ 53,004
Concentration risk, percentage			6.50%	22.70%
Gross Premiums Written | Customer Concentration Risk | Marsh Ltd
Revenue, Major Customer [Line Items]
Gross premiums written			$ 56,781	$ 57,047
Concentration risk, percentage			14.30%	24.50%
Gross Premiums Written | Customer Concentration Risk | Other
Revenue, Major Customer [Line Items]
Gross premiums written			$ 46,992	$ 30,022
Concentration risk, percentage			11.80%	12.90%
Gross Premiums Written | Customer Concentration Risk | Stonehill Reinsurance Partners, LLC
Revenue, Major Customer [Line Items]
Gross premiums written			$ 34,875	$ 46,659
Concentration risk, percentage			8.80%	20.00%
Gross Premiums Written | Customer Concentration Risk | Guy Carpenter & Company, LLC
Revenue, Major Customer [Line Items]
Gross premiums written			$ 7,986	$ 15,074
Concentration risk, percentage			2.00%	6.50%
Gross Premiums Written | Customer Concentration Risk | Tiger Risk Partners LLC [Member]
Revenue, Major Customer [Line Items]
Gross premiums written			$ 7,590	$ 14,496
Concentration risk, percentage			1.90%	6.20%