Segment reporting - Geographic information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Gross premiums written	$ 80,779	$ 142,573	$ 196,866	$ 197,156	$ 99,155	$ 205,583	$ 184,342	$ 213,334	$ 617,374	$ 702,414	$ 613,300
Gross Premiums Written
Revenue from External Customer [Line Items]
Gross premiums written									$ 617,374	$ 702,414	$ 613,300
Concentration risk, percentage									100.00%	100.00%	100.00%
Gross Premiums Written | United States
Revenue from External Customer [Line Items]
Gross premiums written									$ 332,849	$ 283,626	$ 339,061
Concentration risk, percentage									53.90%	40.40%	55.30%
Gross Premiums Written | United Kingdom
Revenue from External Customer [Line Items]
Gross premiums written									$ 187,625	$ 290,710	$ 176,522
Concentration risk, percentage									30.40%	41.40%	28.80%
Gross Premiums Written | Bermuda
Revenue from External Customer [Line Items]
Gross premiums written									$ 96,900	$ 128,078	$ 97,717
Concentration risk, percentage									15.70%	18.20%	15.90%