Segment reporting - Concentration Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue, Major Customer [Line Items]
Reinsurance balances receivable	$ 381,951	$ 294,313
Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%
Contract Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%
Contract Concentration Risk | Gross Premiums Written | Contract 1
Revenue, Major Customer [Line Items]
Concentration risk, percentage	16.10%	16.10%	20.40%
Contract Concentration Risk | Gross Premiums Written | Contract 2
Revenue, Major Customer [Line Items]
Concentration risk, percentage		13.00%	17.10%
Contract Concentration Risk | Gross Premiums Written | Total for contracts contributing greater than 10% each
Revenue, Major Customer [Line Items]
Concentration risk, percentage	16.10%	29.10%	37.50%
Contract Concentration Risk | Gross Premiums Written | Total for contracts contributing less than 10% each
Revenue, Major Customer [Line Items]
Concentration risk, percentage	83.90%	70.90%	62.50%
Customer Concentration Risk | Gross Premiums Written
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%	100.00%
Customer Concentration Risk | Reinsurance Receivables
Revenue, Major Customer [Line Items]
Concentration risk, percentage	100.00%	100.00%
Reinsurance balances receivable	$ 381,951	$ 294,313
Customer Concentration Risk | Reinsurance Receivables | Counterparty 1
Revenue, Major Customer [Line Items]
Concentration risk, percentage	21.50%	29.60%
Reinsurance balances receivable	$ 82,162	$ 87,044
Customer Concentration Risk | Reinsurance Receivables | Other counterparties representing less than 10% each
Revenue, Major Customer [Line Items]
Concentration risk, percentage	78.50%	70.40%
Reinsurance balances receivable	$ 299,789	$ 207,269