Schedule III - Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 39,343	$ 37,792	$ 34,426
Loss and settlement expense reserves	661,309	610,181	583,097
Unearned premiums	232,093	220,627	196,215
Premium revenue	540,722	515,506	458,846
Net investment income	46,465	43,022	44,145
Losses and settlement expenses incurred	385,474	333,287	303,388
Amortization of deferred policy acquisition costs	99,042	94,728	84,275
Other underwriting expenses	56,826	65,754	60,919
Premiums written	552,610	534,077	478,481
Property and casualty insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	33,855	32,740	30,872
Loss and settlement expense reserves	463,458	425,758	412,317
Unearned premiums	204,357	191,714	177,618
Premium revenue	422,381	392,719	357,139
Net investment income	33,509	31,397	32,214
Losses and settlement expenses incurred	298,033	260,917	233,892
Amortization of deferred policy acquisition costs	72,768	68,851	63,641
Other underwriting expenses	54,385	62,522	59,183
Premiums written	433,707	405,049	371,235
Reinsurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	5,488	5,052	3,554
Loss and settlement expense reserves	197,851	184,423	170,780
Unearned premiums	27,736	28,913	18,597
Premium revenue	118,341	122,787	101,707
Net investment income	12,968	11,635	11,941
Losses and settlement expenses incurred	87,441	72,370	69,496
Amortization of deferred policy acquisition costs	26,274	25,877	20,634
Other underwriting expenses	2,441	3,232	1,736
Premiums written	118,903	129,028	107,246
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	(12)	(10)	(10)
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