Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 41,114	$ 40,939	$ 40,720
Loss and settlement expense reserves	732,612	690,532	678,774
Unearned premiums	257,797	244,885	239,435
Premium revenue	607,158	592,408	570,266
Net investment income	45,479	47,490	45,582
Losses and settlement expenses incurred	421,969	386,897	370,736
Amortization of deferred policy acquisition costs	108,910	108,403	102,184
Other underwriting expenses	76,806	69,612	68,418
Premiums written	616,301	594,703	578,938
Property and casualty insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	37,140	36,295	35,219
Loss and settlement expense reserves	502,864	486,387	480,413
Unearned premiums	236,030	220,697	212,652
Premium revenue	472,369	456,467	447,197
Net investment income	32,670	33,886	32,668
Losses and settlement expenses incurred	302,973	294,369	291,883
Amortization of deferred policy acquisition costs	79,734	78,493	75,701
Other underwriting expenses	74,133	66,463	63,954
Premiums written	484,027	463,673	454,434
Reinsurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	3,974	4,644	5,501
Loss and settlement expense reserves	229,748	204,145	198,361
Unearned premiums	21,767	24,188	26,783
Premium revenue	134,789	135,941	123,069
Net investment income	12,771	13,591	12,923
Losses and settlement expenses incurred	118,996	92,528	78,853
Amortization of deferred policy acquisition costs	29,176	29,910	26,483
Other underwriting expenses	2,673	3,149	4,464
Premiums written	132,274	131,030	124,504
Parent company [Member]
Supplementary Insurance Information, by Segment [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	38	13	(9)
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