OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
REVENUES
Net premiums earned	$ 196,522	$ 184,331	$ 386,549	$ 367,616
Net investment income	14,577	13,238	28,809	26,243
Net realized gains	20,849	(1,359)	29,253	(735)
Net unrealized losses on equity securities	(12,611)		(39,383)
Consolidated revenue	219,337	196,210	405,228	393,124
NET EARNINGS
Net underwriting income	14,146	19,743	31,633	32,800
Net investment income	14,577	13,238	28,809	26,243
Net realized gains	20,849	(1,359)	29,253	(735)
Net unrealized losses on equity securities	(12,611)		(39,383)
General corporate expense and interest on debt	(4,499)	(4,392)	(8,638)	(9,573)
Equity in earnings of unconsolidated investees	7,100	6,806	12,266	11,744
Earnings before income taxes	39,562	34,036	53,940	60,479
Income tax expense	6,311	7,828	8,473	14,443
Net earnings	33,251	26,208	45,467	46,036
Casualty segment
REVENUES
Net premiums earned	129,613	119,259	255,463	236,243
NET EARNINGS
Net underwriting income	2,854	8,648	4,415	1,767
Property segment
REVENUES
Net premiums earned	37,190	34,485	72,372	70,290
NET EARNINGS
Net underwriting income	3,647	2,863	9,529	11,667
Surety segment
REVENUES
Net premiums earned	29,719	30,587	58,714	61,083
NET EARNINGS
Net underwriting income	$ 7,645	$ 8,232	$ 17,689	$ 19,366