Income taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets [Abstract]
Unpaid losses and loss adjustment expenses	$ 6,854	$ 7,001
Unearned premiums	4,711	3,748
Organizational costs	196	230
State operating loss carryforwards	708	645
Allowance for doubtful accounts	551	449
Unrealized losses on fixed-maturity securities	837	0
Other	753	431
Deferred tax assets before allowance	14,610	12,504
Less: valuation allowance	(780)	(690)
Total deferred tax assets	13,830	11,814
Deferred tax liabilities [Abstract]
Unrealized gains on fixed-maturity securities	0	616
deferred tax liabilities, unrealized gains on equity securities	374	1,759
Deferred policy acquisition costs, net of ceding commissions	3,108	2,473
Intangible assets	743	743
Transition adjustment for loss discounting	2,060	3,524
Other	369	207
Total deferred tax liabilities	6,654	9,322
Net deferred tax assets	7,176	2,492
State Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 14,900	$ 13,600