OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
REVENUES
Net premiums earned	$ 208,734		$ 207,541		$ 424,316	$ 412,230
Net investment income	16,917		16,998		34,695	33,563
Net realized gains (losses)	(2,109)		4,764		13,043	13,832
Net unrealized gains (losses) on equity securities	74,705		8,810		(55,690)	42,308
Consolidated revenue	298,247		238,113		416,364	501,933
NET EARNINGS
Net underwriting income	24,163		14,932		41,402	37,365
Net investment income	16,917		16,998		34,695	33,563
Net realized gains (losses)	(2,109)		4,764		13,043	13,832
Net unrealized gains (losses) on equity securities	74,705		8,810		(55,690)	42,308
General corporate expense and interest on debt	(3,897)		(5,144)		(7,549)	(10,281)
Equity in earnings of unconsolidated investees	5,100		8,468		9,614	13,782
Earnings before income taxes	114,879		48,828		35,515	130,569
Income tax expense	22,713		8,361		4,616	24,629
Net earnings	92,166	$ (61,267)	40,467	$ 65,473	30,899	105,940
Casualty segment
REVENUES
Net premiums earned	135,215		138,344		278,635	275,244
NET EARNINGS
Net underwriting income	9,733		6,939		8,410	12,282
Property segment
REVENUES
Net premiums earned	45,387		39,972		89,735	78,718
NET EARNINGS
Net underwriting income	6,167		(436)		16,075	7,810
Surety segment
REVENUES
Net premiums earned	28,132		29,225		55,946	58,268
NET EARNINGS
Net underwriting income	$ 8,263		$ 8,429		$ 16,917	$ 17,273