Underwriting, acquisition and insurance expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Underwriting, Acquisition and Insurance Expenses [Abstract]
Direct commissions	$ 31,001	$ 26,715	$ 25,241
Ceding commissions	(9,799)	(20,568)	(42,081)
Other expenses	22,944	22,404	19,649
Total	44,146	28,551	2,809
Salaries, employee benefits and bonus expense	$ 17,700	$ 18,700	$ 15,500