Income taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets [Abstract]
Unpaid losses and loss adjustment expenses	$ 7,001	$ 4,528
Unearned premiums	3,748	5,308
Organizational costs	230	1,513
State operating loss carryforwards	645	476
Allowance for doubtful accounts	449	693
Other	431	438
Deferred tax assets before allowance	12,504	12,956
Less: valuation allowance	(690)	(623)
Total deferred tax assets	11,814	12,333
Deferred tax liabilities [Abstract]
Unrealized gain on investments	2,375	620
Deferred policy acquisition costs	2,473	3,553
Intangible assets	743	1,238
Transition adjustment for loss discounting	3,524	0
Other	207	317
Total deferred tax liabilities	9,322	5,728
Net deferred tax assets	2,492	6,605
State Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 13,600	$ 12,200