Segment reporting - Geographic information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue from External Customer [Line Items]
Gross premiums written	$ 49,765	$ 156,564	$ 428,125	$ 302,918
Gross Premiums Written
Revenue from External Customer [Line Items]
Gross premiums written	$ 49,765	$ 156,564	$ 428,125	$ 302,918
Concentration risk, percentage	100.00%	100.00%	100.00%	100.00%
Gross Premiums Written | United States
Revenue from External Customer [Line Items]
Gross premiums written	$ 31,400	$ 1,340	$ 264,271	$ 43,769
Concentration risk, percentage	63.10%	0.80%	61.70%	14.40%
Gross Premiums Written | United Kingdom
Revenue from External Customer [Line Items]
Gross premiums written	$ 15,008	$ 150,744	$ 66,993	$ 200,594
Concentration risk, percentage	30.20%	96.30%	15.70%	66.20%
Gross Premiums Written | Bermuda
Revenue from External Customer [Line Items]
Gross premiums written	$ 3,357	$ 0	$ 73,710	$ 54,075
Concentration risk, percentage	6.70%	0.00%	17.20%	17.90%
Gross Premiums Written | Other
Revenue from External Customer [Line Items]
Gross premiums written	$ 0	$ 4,480	$ 23,151	$ 4,480
Concentration risk, percentage	0.00%	2.90%	5.40%	1.50%