OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
REVENUES
Net premiums earned	$ 200,815	$ 182,025	$ 587,364	$ 549,641
Net investment income	16,314	14,187	45,123	40,430
Net realized gains (losses)	18,647	35	47,900	(700)
Net unrealized gains (losses) on equity securities	4,848		(34,535)
Consolidated revenue	240,624	196,247	645,852	589,371
NET EARNINGS
Net underwriting income	7,762	(14,932)	39,395	17,868
Net investment income	16,314	14,187	45,123	40,430
Net realized gains	18,647	35	47,900	(700)
Net unrealized gains (losses) on equity securities	4,848		(34,535)
General corporate expense and interest on debt	(4,809)	(3,812)	(13,447)	(13,385)
Equity in earnings of unconsolidated investees	3,587	3,660	15,853	15,404
Earnings (loss) before income taxes	46,349	(862)	100,289	59,617
Income tax expense	6,977	(2,596)	15,450	11,847
Net earnings	39,372	1,734	84,839	47,770
Casualty segment
REVENUES
Net premiums earned	131,605	118,393	387,068	354,636
NET EARNINGS
Net underwriting income	(1,974)	3,554	2,441	5,321
Property segment
REVENUES
Net premiums earned	39,067	33,559	111,439	103,849
NET EARNINGS
Net underwriting income	2,621	(27,519)	12,150	(15,852)
Surety segment
REVENUES
Net premiums earned	30,143	30,073	88,857	91,156
NET EARNINGS
Net underwriting income	$ 7,115	$ 9,033	$ 24,804	$ 28,399