REINSURANCE (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) reinsurer	Dec. 31, 2016 USD ($) reinsurer	Dec. 31, 2015 USD ($)
Reinsurance Transactions For Insurance Companies, by Product Segment [Line Items]
Number of significant nonaffiliated reinsurers | reinsurer	3	2
Total receivable amount from significant nonaffiliated reinsurers	$ 22,700	$ 10,400
Percentage of pool participation (in hundredths)	30.00%	30.00%	30.00%
Premiums written
Net premiums written	$ 616,301	$ 594,703	$ 578,938
Premiums earned
Direct	384,993	382,300	366,752
Assumed	660,046	637,054	611,045
Ceded	(437,881)	(426,946)	(407,531)
Net premiums earned	607,158	592,408	570,266
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	421,969	386,897	370,736
Affiliated Entity [Member]
Premiums written
Assumed	520,932	491,315	474,323
Ceded	(403,219)	(396,731)	(381,782)
Premiums earned
Assumed	505,795	483,759	466,966
Ceded	(397,183)	(395,220)	(377,579)
Losses and settlement expenses incurred
Assumed	335,570	305,818	295,181
Ceded	(288,023)	(236,851)	(199,126)
Non Affiliated Entity [Member]
Premiums written
Direct	391,029	383,811	370,955
Assumed	151,738	150,780	143,092
Ceded	(44,179)	(34,472)	(27,650)
Premiums earned
Direct	384,993	382,300	366,752
Assumed	154,251	153,295	144,079
Ceded	(40,698)	(31,726)	(29,952)
Losses and settlement expenses incurred
Direct	252,007	229,859	198,504
Assumed	145,566	96,018	85,922
Ceded	$ (23,151)	$ (7,947)	$ (9,745)
Property and casualty insurance [Member]
Reinsurance Transactions For Insurance Companies, by Product Segment [Line Items]
Percentage of pool participation (in hundredths)	30.00%	30.00%	30.00%
Premiums written
Net premiums written	$ 484,027	$ 463,673	$ 454,434
Premiums earned
Net premiums earned	472,369	456,467	447,197
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	$ 302,973	$ 294,369	$ 291,883
Property and casualty insurance [Member] | Affiliated Entity [Member]
Reinsurance Transactions For Insurance Companies, by Product Segment [Line Items]
Percentage of pool participation (in hundredths)	30.00%	30.00%	30.00%
Premiums written
Assumed	$ 520,932	$ 491,315	$ 474,323
Ceded	(398,369)	(391,651)	(370,955)
Premiums earned
Assumed	505,795	483,759	466,966
Ceded	(392,333)	(390,140)	(366,752)
Losses and settlement expenses incurred
Assumed	334,240	304,007	294,324
Ceded	(271,185)	(237,318)	(198,504)
Property and casualty insurance [Member] | Non Affiliated Entity [Member]
Premiums written
Direct	391,029	383,811	370,955
Assumed	4,454	4,544	4,392
Ceded	(34,019)	(24,346)	(24,281)
Premiums earned
Direct	384,993	382,300	366,752
Assumed	4,299	4,444	4,240
Ceded	(30,385)	(23,896)	(24,009)
Losses and settlement expenses incurred
Direct	252,007	229,859	198,504
Assumed	2,879	2,712	2,407
Ceded	(14,968)	(4,891)	(4,848)
Reinsurance [Member]
Premiums written
Net premiums written	132,274	131,030	124,504
Premiums earned
Net premiums earned	134,789	135,941	123,069
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	118,996	92,528	78,853
Reinsurance [Member] | Affiliated Entity [Member]
Premiums written
Assumed	0	0	0
Ceded	(4,850)	(5,080)	(10,827)
Premiums earned
Assumed	0	0	0
Ceded	(4,850)	(5,080)	(10,827)
Losses and settlement expenses incurred
Assumed	1,330	1,811	857
Ceded	(16,838)	467	(622)
Reinsurance [Member] | Non Affiliated Entity [Member]
Premiums written
Direct	0	0	0
Assumed	147,284	146,236	138,700
Ceded	(10,160)	(10,126)	(3,369)
Premiums earned
Direct	0	0	0
Assumed	149,952	148,851	139,839
Ceded	(10,313)	(7,830)	(5,943)
Losses and settlement expenses incurred
Direct	0	0	0
Assumed	142,687	93,306	83,515
Ceded	$ (8,183)	$ (3,056)	$ (4,897)