Segment reporting - Customer Concentration (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenue, Major Customer [Line Items]
Gross premiums written	$ 45,425	$ 41,651	$ 239,660	$ 162,479
Gross Premiums Written
Revenue, Major Customer [Line Items]
Gross premiums written			239,660	162,479
Concentration risk, percentage			100.00%	100.00%
Gross Premiums Written | Customer Concentration Risk
Revenue, Major Customer [Line Items]
Gross premiums written			163,444	82,150
Concentration risk, percentage			68.10%	50.60%
Gross Premiums Written | Customer Concentration Risk | Largest broker
Revenue, Major Customer [Line Items]
Gross premiums written			91,349	38,117
Concentration risk, percentage			38.10%	23.50%
Gross Premiums Written | Customer Concentration Risk | 2nd largest broker
Revenue, Major Customer [Line Items]
Gross premiums written			46,095	22,473
Concentration risk, percentage			19.20%	13.80%
Gross Premiums Written | Customer Concentration Risk | 3rd largest broker
Revenue, Major Customer [Line Items]
Gross premiums written			$ 26,000	$ 21,560
Concentration risk, percentage			10.80%	13.30%