SEGMENT INFORMATION - Summarized financial information by segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									$ 607,158	$ 592,408	$ 570,266
SAP underwriting profit (loss)									(12,684)	15,653	15,722
GAAP adjustments									4,547	(1,957)	5,501
GAAP underwriting profit (loss)									(8,137)	13,696	21,223
Net investment income (loss)									45,479	47,490	45,582
Net realized investment gains (losses)									6,556	4,074	6,153
Other income (loss)									(348)	1,011	1,725
Interest expense									337	337	337
Other expenses									3,397	2,727	2,690
Income before income tax expense	$ 24,587	$ (440)	$ 7,229	$ 8,440	$ 29,196	$ 5,047	$ 8,087	$ 20,877	39,816	63,207	71,656
Reconciliation of assets from segment to consolidated
Assets	2,289,419				2,131,694				2,289,419	2,131,694
Eliminations	(599,036)				(540,249)				(599,036)	(540,249)
Reclassifications	(8,443)				(2,632)				(8,443)	(2,632)
Total assets	1,681,940				1,588,813				1,681,940	1,588,813
Property and casualty insurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									472,369	456,467	447,197
SAP underwriting profit (loss)									2,702	4,276	2,494
GAAP adjustments									5,217	(934)	5,460
GAAP underwriting profit (loss)									7,919	3,342	7,954
Net investment income (loss)									32,670	33,886	32,668
Net realized investment gains (losses)									4,896	4,082	4,163
Other income (loss)									1,171	594	771
Interest expense									337	337	337
Other expenses									1,128	721	748
Income before income tax expense									45,191	40,846	44,471
Reconciliation of assets from segment to consolidated
Assets	1,200,636				1,122,037				1,200,636	1,122,037
Eliminations	0				0				0	0
Reclassifications	(1,393)				0				(1,393)	0
Total assets	1,199,243				1,122,037				1,199,243	1,122,037
Reinsurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									134,789	135,941	123,069
SAP underwriting profit (loss)									(15,386)	11,377	13,228
GAAP adjustments									(670)	(1,023)	41
GAAP underwriting profit (loss)									(16,056)	10,354	13,269
Net investment income (loss)									12,771	13,591	12,923
Net realized investment gains (losses)									1,660	(8)	1,990
Other income (loss)									(1,519)	417	954
Interest expense									0	0	0
Other expenses									0	0	0
Income before income tax expense									(3,144)	24,354	29,136
Reconciliation of assets from segment to consolidated
Assets	484,678				455,493				484,678	455,493
Eliminations	0				0				0	0
Reclassifications	(6,273)				(1,932)				(6,273)	(1,932)
Total assets	478,405				453,561				478,405	453,561
Parent company [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									0	0	0
SAP underwriting profit (loss)									0	0	0
GAAP adjustments									0	0	0
GAAP underwriting profit (loss)									0	0	0
Net investment income (loss)									38	13	(9)
Net realized investment gains (losses)									0	0	0
Other income (loss)									0	0	0
Interest expense									0	0	0
Other expenses									2,269	2,006	1,942
Income before income tax expense									(2,231)	(1,993)	$ (1,951)
Reconciliation of assets from segment to consolidated
Assets	604,105				554,164				604,105	554,164
Eliminations	(599,036)				(540,249)				(599,036)	(540,249)
Reclassifications	(777)				(700)				(777)	(700)
Total assets	$ 4,292				$ 13,215				$ 4,292	$ 13,215