Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	$ 44,760	$ 41,114	$ 40,939
Loss and settlement expense reserves	777,190	732,612	690,532
Unearned premiums	268,511	257,797	244,885
Premium revenue	645,183	607,158	592,408
Net investment income	47,637	45,479	47,490
Losses and settlement expenses incurred	457,159	421,969	386,897
Amortization of deferred policy acquisition costs	115,803	108,910	108,403
Other underwriting expenses	88,824	81,918	74,421
Premiums written	661,043	616,301	594,703
Property and casualty insurance [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	40,674	37,140	36,295
Loss and settlement expense reserves	531,912	502,864	486,387
Unearned premiums	246,330	236,030	220,697
Premium revenue	495,447	472,369	456,467
Net investment income	34,070	32,670	33,886
Losses and settlement expenses incurred	332,921	302,973	294,369
Amortization of deferred policy acquisition costs	83,869	79,734	78,493
Other underwriting expenses	85,967	79,245	71,272
Premiums written	510,525	484,027	463,673
Reinsurance [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	4,086	3,974	4,644
Loss and settlement expense reserves	245,278	229,748	204,145
Unearned premiums	22,181	21,767	24,188
Premium revenue	149,736	134,789	135,941
Net investment income	13,523	12,771	13,591
Losses and settlement expenses incurred	124,238	118,996	92,528
Amortization of deferred policy acquisition costs	31,934	29,176	29,910
Other underwriting expenses	2,857	2,673	3,149
Premiums written	150,518	132,274	131,030
Parent company [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	0	0	0
Loss and settlement expense reserves	0	0	0
Unearned premiums	0	0	0
Premium revenue	0	0	0
Net investment income	44	38	13
Losses and settlement expenses incurred	0	0	0
Amortization of deferred policy acquisition costs	0	0	0
Other underwriting expenses	0	0	0
Premiums written	$ 0	$ 0	$ 0