OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
REVENUES
Net premiums earned	$ 184,331	$ 180,226	$ 367,616	$ 357,144
Net investment income	13,238	13,048	26,243	26,418
Net realized gains (losses)	(1,359)	2,710	(735)	14,110
Consolidated revenue	196,210	195,984	393,124	397,672
NET EARNINGS
Net underwriting income	19,743	26,016	32,800	47,320
Net investment income	13,238	13,048	26,243	26,418
Net realized gains (losses)	(1,359)	2,710	(735)	14,110
General corporate expense and interest on debt	(4,392)	(4,624)	(9,573)	(8,856)
Equity in earnings of unconsolidated investees	6,806	5,191	11,744	8,942
Earnings before income taxes	34,036	42,341	60,479	87,934
Income tax expense	7,828	13,264	14,443	27,464
Net earnings	26,208	29,077	46,036	60,470
Casualty segment
REVENUES
Net premiums earned	119,259	112,360	236,243	220,953
NET EARNINGS
Net underwriting income	8,648	11,125	1,767	20,231
Property segment
REVENUES
Net premiums earned	34,485	37,794	70,290	76,479
NET EARNINGS
Net underwriting income	2,863	7,245	11,667	13,443
Surety segment
REVENUES
Net premiums earned	30,587	30,072	61,083	59,712
NET EARNINGS
Net underwriting income	$ 8,232	$ 7,646	$ 19,366	$ 13,646