Segment reporting - Geographic information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue from External Customer [Line Items]
Gross premiums written	$ 174,539	$ 142,573	$ 477,457	$ 536,595
Gross Premiums Written
Revenue from External Customer [Line Items]
Gross premiums written	$ 174,539	$ 142,573	$ 477,457	$ 536,595
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	$ 145,313	$ 123,233	$ 189,082	$ 290,971
Concentration risk, percentage	83.30%	86.40%	39.60%	54.20%
Gross Premiums Written | United Kingdom
Revenue from External Customer [Line Items]
Gross premiums written	$ 948	$ 802	$ 201,542	$ 180,546
Concentration risk, percentage	0.50%	0.60%	42.20%	33.70%
Gross Premiums Written | Bermuda
Revenue from External Customer [Line Items]
Gross premiums written	$ 8,278	$ 18,538	$ 62,353	$ 65,078
Concentration risk, percentage	4.70%	13.00%	13.10%	12.10%
Gross Premiums Written | Other Geographical Location [Member]
Revenue from External Customer [Line Items]
Gross premiums written	$ 20,000	$ 0	$ 24,480	$ 0
Concentration risk, percentage	11.50%	0.00%	5.10%	0.00%