Segment reporting - Customer Concentration (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Revenue, Major Customer [Line Items]
Gross premiums written	$ 87,587	$ 96,020
Gross Premiums Written
Revenue, Major Customer [Line Items]
Gross premiums written	87,587	96,020
Concentration risk, percentage	100.00%	100.00%
Gross Premiums Written | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Gross premiums written	87,587	96,020
Concentration risk, percentage	100.00%	100.00%
Gross Premiums Written | Customer Concentration Risk | Stonehill Reinsurance Partners, LLC [Member]
Revenue, Major Customer [Line Items]
Gross premiums written	44,744	0
Concentration risk, percentage	51.10%	0.00%
Gross Premiums Written | Customer Concentration Risk | Aon Benfield - a division of Aon plc
Revenue, Major Customer [Line Items]
Gross premiums written	5,996	49,904
Concentration risk, percentage	6.80%	52.00%
Gross Premiums Written | Customer Concentration Risk | Guy Carpenter & Company, LLC
Revenue, Major Customer [Line Items]
Gross premiums written	(414)	1,564
Concentration risk, percentage	(0.50%)	1.60%
Gross Premiums Written | Customer Concentration Risk | Advocate Reinsurance Partners, LLC
Revenue, Major Customer [Line Items]
Gross premiums written	8,455	(640)
Concentration risk, percentage	9.70%	(0.70%)
Gross Premiums Written | Customer Concentration Risk | BMS Intermediaries
Revenue, Major Customer [Line Items]
Gross premiums written	0	8,213
Concentration risk, percentage	0.00%	8.60%
Gross Premiums Written | Customer Concentration Risk | Other brokers
Revenue, Major Customer [Line Items]
Gross premiums written	2,622	3,842
Concentration risk, percentage	3.00%	4.00%
Gross Premiums Written | Customer Concentration Risk | Total broker placed
Revenue, Major Customer [Line Items]
Gross premiums written	61,403	62,883
Concentration risk, percentage	70.10%	65.50%
Gross Premiums Written | Customer Concentration Risk | Other
Revenue, Major Customer [Line Items]
Gross premiums written	$ 26,184	$ 33,137
Concentration risk, percentage	29.90%	34.50%