RESERVES FOR LOSSES AND LOSS SETTLEMENT EXPENSES (Cumulative Paid Claims and Allocated Claim Adjustment Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Claims and Claims Adjustment Expense [Line Items]
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	$ 1,257,183
Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	692,533
All outstanding liabilities for unpaid losses and loss settlement expenses before 2010, net of reinsurance	33,613
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	532,153
Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	1,143,283
All outstanding liabilities for unpaid losses and loss settlement expenses before 2010, net of reinsurance	702
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	113,569
Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	1,142,414
All outstanding liabilities for unpaid losses and loss settlement expenses before 2010, net of reinsurance	23
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	410,716
Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	371,068
All outstanding liabilities for unpaid losses and loss settlement expenses before 2010, net of reinsurance	18,136
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	142,587
Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	440,882
All outstanding liabilities for unpaid losses and loss settlement expenses before 2010, net of reinsurance	841
Liabilities for unpaid losses and loss settlement expenses, net of reinsurance	29,390
2010 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	64,476	$ 63,300	$ 62,070	$ 60,116	$ 56,761	$ 51,336	$ 35,043	$ 24,577	$ 15,230	$ 7,103
2010 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	84,008	83,942	83,915	83,374	83,000	82,037	80,352	78,284	72,271	52,660
2010 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	67,863	67,853	67,852	67,622	65,169	62,231	57,947	52,953	41,141	29,329
2010 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	39,639	39,208	38,973	35,052	34,143	33,119	31,887	28,765	22,606	11,821
2010 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	33,162	33,154	33,158	33,165	32,997	32,771	32,076	31,340	29,867	$ 24,499
2011 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	66,500	64,541	62,165	56,150	50,146	40,595	26,260	13,670	6,236
2011 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	121,063	120,731	120,178	118,183	116,614	112,497	109,429	104,800	85,585
2011 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	90,515	89,459	88,609	87,475	85,348	79,383	65,021	50,931	34,332
2011 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	29,890	29,700	29,453	29,022	28,247	27,497	26,033	21,678	10,322
2011 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	46,752	46,650	46,575	46,573	46,487	45,949	45,306	43,801	$ 36,489
2012 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	73,819	71,773	69,800	64,574	55,316	39,948	24,620	6,875
2012 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	107,992	106,740	106,521	105,250	103,197	100,078	94,380	71,008
2012 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	94,983	94,747	93,179	90,292	82,944	71,469	57,201	39,247
2012 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	36,060	35,388	34,330	33,063	30,933	28,397	23,023	11,802
2012 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	44,158	44,158	44,139	43,569	44,448	43,375	41,979	$ 30,415
2013 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	75,961	72,115	65,773	54,559	39,953	25,228	9,835
2013 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	92,012	91,493	89,200	86,115	82,853	78,226	59,331
2013 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	101,580	100,058	96,375	90,780	79,663	67,630	43,592
2013 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	47,572	47,071	45,078	42,941	38,023	30,209	14,136
2013 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	36,397	36,323	36,155	35,306	34,297	32,788	$ 25,505
2014 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	93,060	83,109	70,363	50,211	29,679	10,207
2014 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	125,745	123,697	122,370	116,750	113,663	84,456
2014 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	113,751	109,682	99,922	87,590	68,033	45,704
2014 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	45,825	45,193	42,964	38,441	30,289	13,965
2014 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	52,543	52,018	51,837	49,710	47,912	$ 37,055
2015 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	96,339	74,292	53,901	27,182	11,185
2015 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	104,115	101,367	95,515	90,454	67,217
2015 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	129,317	118,395	99,201	78,225	50,782
2015 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	39,305	38,424	35,229	27,304	12,063
2015 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	39,865	39,428	39,027	37,431	$ 29,551
2016 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	88,885	63,526	38,184	13,782
2016 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	137,909	132,429	125,962	92,895
2016 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	148,224	128,157	103,528	66,013
2016 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	45,743	40,680	32,345	14,413
2016 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	44,046	42,660	40,910	$ 32,999
2017 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	70,500	43,172	17,716
2017 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	145,900	137,058	99,484
2017 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	166,170	126,644	81,311
2017 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	38,083	31,309	14,647
2017 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	55,982	53,111	$ 42,135
2018 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	44,772	16,200
2018 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	123,559	92,770
2018 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	138,092	81,572
2018 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	35,369	16,949
2018 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	47,433	$ 37,410
2019 | Commercial other liability
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	18,221
2019 | Commercial fire and allied
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	100,980
2019 | Commercial automobile
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	91,919
2019 | Workers' compensation
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	13,582
2019 | Personal
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative paid losses and allocated loss settlement expenses, net of reinsurance	$ 40,544