Underwriting, acquisition and insurance expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Underwriting, Acquisition and Insurance Expenses [Abstract]
Direct commissions	$ 36,885	$ 31,001	$ 26,715
Ceding commissions	(10,448)	(9,799)	(20,568)
Other expenses	26,988	22,944	22,404
Total	53,425	44,146	28,551
Salaries, employee benefits and bonus expense	$ 19,700	$ 17,700	$ 18,700