SEGMENT INFORMATION - Summarized financial information by segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									$ 570,266	$ 540,722	$ 515,506
Underwriting profit (loss)									21,223	(10,124)	10,873
Net investment income (loss)									45,582	46,465	43,022
Net realized investment gains									6,153	4,349	8,997
Other income (loss)									1,725	2,931	460
Interest expense									337	337	384
Other expenses									2,690	2,377	2,115
Income before income tax expense	$ 13,923	$ 15,921	$ 11,875	$ 29,937	$ 23,865	$ 1,883	$ 270	$ 14,889	71,656	40,907	60,853
Reconciliation of assets from segment to consolidated
Assets	2,055,437				1,994,576				2,055,437	1,994,576
Eliminations	(514,309)				(495,288)				(514,309)	(495,288)
Reclassifications	(5,173)				(1,468)				(5,173)	(1,468)
Total assets	1,535,955				1,497,820				1,535,955	1,497,820
Property and casualty insurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									447,197	422,381	392,719
Underwriting profit (loss)									7,954	(12,309)	(10,435)
Net investment income (loss)									32,668	33,509	31,397
Net realized investment gains									4,163	2,938	7,525
Other income (loss)									771	695	765
Interest expense									337	337	384
Other expenses									748	793	751
Income before income tax expense									44,471	23,703	28,117
Reconciliation of assets from segment to consolidated
Assets	1,092,820				1,057,429				1,092,820	1,057,429
Eliminations	0				0				0	0
Reclassifications	0				(909)				0	(909)
Total assets	1,092,820				1,056,520				1,092,820	1,056,520
Reinsurance [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									123,069	118,341	122,787
Underwriting profit (loss)									13,269	2,185	21,308
Net investment income (loss)									12,923	12,968	11,635
Net realized investment gains									1,990	1,411	1,472
Other income (loss)									954	2,236	(305)
Interest expense									0	0	0
Other expenses									0	0	0
Income before income tax expense									29,136	18,800	34,110
Reconciliation of assets from segment to consolidated
Assets	437,575				434,139				437,575	434,139
Eliminations	0				0				0	0
Reclassifications	(5,173)				0				(5,173)	0
Total assets	432,402				434,139				432,402	434,139
Parent company [Member]
Reconciliation of operating profit loss from segment to consolidated
Premiums earned									0	0	0
Underwriting profit (loss)									0	0	0
Net investment income (loss)									(9)	(12)	(10)
Net realized investment gains									0	0	0
Other income (loss)									0	0	0
Interest expense									0	0	0
Other expenses									1,942	1,584	1,364
Income before income tax expense									(1,951)	(1,596)	$ (1,374)
Reconciliation of assets from segment to consolidated
Assets	525,042				503,008				525,042	503,008
Eliminations	(514,309)				(495,288)				(514,309)	(495,288)
Reclassifications	0				(559)				0	(559)
Total assets	$ 10,733				$ 7,161				$ 10,733	$ 7,161