SEGMENT INFORMATION - Summarized financial information by segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									$ 645,183	$ 607,158	$ 592,408
SAP underwriting profit (loss)									(24,581)	(12,684)	15,653
GAAP adjustments									(1,231)	(565)	(6,766)
GAAP underwriting profit (loss)									(25,812)	(13,249)	8,887
Net investment income									47,637	45,479	47,490
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments									(41,252)	6,556	4,074
Other income (loss)									9,159	4,764	5,820
Interest expense									654	337	337
Other expenses									3,754	3,397	2,727
Income (loss) before income tax expense (benefit)	$ (30,870)	$ 24,444	$ (7,722)	$ (528)	$ 24,587	$ (440)	$ 7,229	$ 8,440	(14,676)	39,816	63,207
Reconciliation of assets from segment to consolidated
Assets	2,242,461				2,289,419				2,242,461	2,289,419
Eliminations	(556,977)				(599,036)				(556,977)	(599,036)
Reclassifications	(6)				(8,443)				(6)	(8,443)
Total assets	1,685,478				1,681,940				1,685,478	1,681,940
Property and casualty insurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									495,447	472,369	456,467
SAP underwriting profit (loss)									(15,177)	2,702	4,276
GAAP adjustments									(1,342)	105	(5,743)
GAAP underwriting profit (loss)									(16,519)	2,807	(1,467)
Net investment income									34,070	32,670	33,886
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments									(28,227)	4,896	4,082
Other income (loss)									8,444	6,283	5,403
Interest expense									654	337	337
Other expenses									1,202	1,128	721
Income (loss) before income tax expense (benefit)									(4,088)	45,191	40,846
Reconciliation of assets from segment to consolidated
Assets	1,191,286				1,200,636				1,191,286	1,200,636
Eliminations	0				0				0	0
Reclassifications	0				(1,393)				0	(1,393)
Total assets	1,191,286				1,199,243				1,191,286	1,199,243
Reinsurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									149,736	134,789	135,941
SAP underwriting profit (loss)									(9,404)	(15,386)	11,377
GAAP adjustments									111	(670)	(1,023)
GAAP underwriting profit (loss)									(9,293)	(16,056)	10,354
Net investment income									13,523	12,771	13,591
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments									(12,935)	1,660	(8)
Other income (loss)									715	(1,519)	417
Interest expense									0	0	0
Other expenses									0	0	0
Income (loss) before income tax expense (benefit)									(7,990)	(3,144)	24,354
Reconciliation of assets from segment to consolidated
Assets	485,270				484,678				485,270	484,678
Eliminations	0				0				0	0
Reclassifications	0				(6,273)				0	(6,273)
Total assets	485,270				478,405				485,270	478,405
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									44	38	13
Net realized investment gains (losses) and, beginning in 2018, change in unrealized investment gains on equity investments									(90)	0	0
Other income (loss)									0	0	0
Interest expense									0	0	0
Other expenses									2,552	2,269	2,006
Income (loss) before income tax expense (benefit)									(2,598)	(2,231)	$ (1,993)
Reconciliation of assets from segment to consolidated
Assets	565,905				604,105				565,905	604,105
Eliminations	(556,977)				(599,036)				(556,977)	(599,036)
Reclassifications	(6)				(777)				(6)	(777)
Total assets	$ 8,922				$ 4,292				$ 8,922	$ 4,292