OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
REVENUES
Net premiums earned	$ 207,541		$ 196,522		$ 412,230	$ 386,549
Net investment income	16,998		14,577		33,563	28,809
Net realized gains	4,764		20,849		13,832	29,253
Net unrealized gains (losses) on equity securities	8,810		(12,611)		42,308	(39,383)
Consolidated revenue	238,113		219,337		501,933	405,228
NET EARNINGS
Net underwriting income	14,932		14,146		37,365	31,633
Net investment income	16,998		14,577		33,563	28,809
Net realized gains	4,764		20,849		13,832	29,253
Net unrealized gains (losses) on equity securities	8,810		(12,611)		42,308	(39,383)
General corporate expense and interest on debt	(5,144)		(4,499)		(10,281)	(8,638)
Equity in earnings of unconsolidated investees	8,468		7,100		13,782	12,266
Earnings before income taxes	48,828		39,562		130,569	53,940
Income tax expense	8,361		6,311		24,629	8,473
Net earnings	40,467	$ 65,473	33,251	$ 12,216	105,940	45,467
Casualty segment
REVENUES
Net premiums earned	138,344		129,613		275,244	255,463
NET EARNINGS
Net underwriting income	6,939		2,854		12,282	4,415
Property segment
REVENUES
Net premiums earned	39,972		37,190		78,718	72,372
NET EARNINGS
Net underwriting income	(436)		3,647		7,810	9,529
Surety segment
REVENUES
Net premiums earned	29,225		29,719		58,268	58,714
NET EARNINGS
Net underwriting income	$ 8,429		$ 7,645		$ 17,273	$ 17,689