OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
REVENUES
Net premiums earned	$ 204,689	$ 190,027
Net investment income	16,565	14,232
Net realized gains	9,068	8,404
Net unrealized gains (losses) on equity securities	33,498	(26,772)
Consolidated revenue	263,820	185,891
NET EARNINGS
Net underwriting income	22,433	17,487
Net investment income	16,565	14,232
Net realized gains	9,068	8,404
Net unrealized gains (losses) on equity securities	33,498	(26,772)
General corporate expense and interest on debt	(5,137)	(4,139)
Equity in earnings of unconsolidated investees	5,314	5,166
Earnings before income taxes	81,741	14,378
Income tax expense	16,268	2,162
Net earnings	65,473	12,216
Casualty segment
REVENUES
Net premiums earned	136,900	125,850
NET EARNINGS
Net underwriting income	5,343	1,561
Property segment
REVENUES
Net premiums earned	38,746	35,182
NET EARNINGS
Net underwriting income	8,246	5,882
Surety segment
REVENUES
Net premiums earned	29,043	28,995
NET EARNINGS
Net underwriting income	$ 8,844	$ 10,044