REINSURANCE (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) reinsurer	Dec. 31, 2017 USD ($) reinsurer	Dec. 31, 2016 USD ($)
Reinsurance Transactions For Insurance Companies, by Product Segment [Line Items]
Number of significant nonaffiliated reinsurers | reinsurer	2	3
Total receivable amount from significant nonaffiliated reinsurers	$ 16,600	$ 22,700
Percentage of pool participation (in hundredths)	30.00%	30.00%	30.00%
Premiums written
Net premiums written	$ 661,043	$ 616,301	$ 594,703
Premiums earned
Direct	399,660	384,993	382,300
Assumed	700,918	660,046	637,054
Ceded	(455,395)	(437,881)	(426,946)
Net premiums earned	645,183	607,158	592,408
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	457,159	421,969	386,897
Affiliated Entity [Member]
Premiums written
Assumed	542,480	520,932	491,315
Ceded	(421,411)	(403,219)	(396,731)
Premiums earned
Assumed	531,384	505,795	483,759
Ceded	(412,250)	(397,183)	(395,220)
Losses and settlement expenses incurred
Assumed	353,729	335,570	305,818
Ceded	(226,221)	(288,023)	(236,851)
Non Affiliated Entity [Member]
Premiums written
Direct	408,821	391,029	383,811
Assumed	170,297	151,738	150,780
Ceded	(39,144)	(44,179)	(34,472)
Premiums earned
Direct	399,660	384,993	382,300
Assumed	169,534	154,251	153,295
Ceded	(43,145)	(40,698)	(31,726)
Losses and settlement expenses incurred
Direct	225,676	252,007	229,859
Assumed	131,824	145,566	96,018
Ceded	$ (27,849)	$ (23,151)	$ (7,947)
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	$ 510,525	$ 484,027	$ 463,673
Premiums earned
Net premiums earned	495,447	472,369	456,467
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	$ 332,921	$ 302,973	$ 294,369
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	$ 542,480	$ 520,932	$ 491,315
Ceded	(416,161)	(398,369)	(391,651)
Premiums earned
Assumed	531,384	505,795	483,759
Ceded	(407,000)	(392,333)	(390,140)
Losses and settlement expenses incurred
Assumed	352,423	334,240	304,007
Ceded	(230,848)	(271,185)	(237,318)
Property and casualty insurance [Member] | Non Affiliated Entity [Member]
Premiums written
Direct	408,821	391,029	383,811
Assumed	4,550	4,454	4,544
Ceded	(29,165)	(34,019)	(24,346)
Premiums earned
Direct	399,660	384,993	382,300
Assumed	4,482	4,299	4,444
Ceded	(33,079)	(30,385)	(23,896)
Losses and settlement expenses incurred
Direct	225,676	252,007	229,859
Assumed	3,470	2,879	2,712
Ceded	(17,800)	(14,968)	(4,891)
Reinsurance [Member]
Premiums written
Net premiums written	150,518	132,274	131,030
Premiums earned
Net premiums earned	149,736	134,789	135,941
Losses and settlement expenses incurred
Net losses and settlement expenses incurred	124,238	118,996	92,528
Reinsurance [Member] | Affiliated Entity [Member]
Premiums written
Assumed	0	0	0
Ceded	(5,250)	(4,850)	(5,080)
Premiums earned
Assumed	0	0	0
Ceded	(5,250)	(4,850)	(5,080)
Losses and settlement expenses incurred
Assumed	1,306	1,330	1,811
Ceded	4,627	(16,838)	467
Reinsurance [Member] | Non Affiliated Entity [Member]
Premiums written
Direct	0	0	0
Assumed	165,747	147,284	146,236
Ceded	(9,979)	(10,160)	(10,126)
Premiums earned
Direct	0	0	0
Assumed	165,052	149,952	148,851
Ceded	(10,066)	(10,313)	(7,830)
Losses and settlement expenses incurred
Direct	0	0	0
Assumed	128,354	142,687	93,306
Ceded	$ (10,049)	$ (8,183)	$ (3,056)