OPERATING SEGMENT INFORMATION - Reconciliation of Segment Totals to Total Earnings (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
REVENUES
Net premiums earned	$ 182,025	$ 183,595	$ 549,641	$ 540,739
Net investment income	14,187	13,504	40,430	39,922
Net realized gains (losses)	35	9,252	(700)	23,362
Consolidated revenue	196,247	206,351	589,371	604,023
NET EARNINGS
Net underwriting income	(14,932)	10,604	17,868	57,924
Net investment income	14,187	13,504	40,430	39,922
Net realized gains (losses)	35	9,252	(700)	23,362
General corporate expense and interest on debt	(3,812)	(4,099)	(13,385)	(12,955)
Equity in earnings of unconsolidated investees	3,660	1,881	15,404	10,823
Earnings (loss) before income taxes	(862)	31,142	59,617	119,076
Income tax expense	(2,596)	8,879	11,847	36,343
Net earnings	1,734	22,263	47,770	82,733
Casualty segment
REVENUES
Net premiums earned	118,393	115,619	354,636	336,572
NET EARNINGS
Net underwriting income	3,554	259	5,321	20,490
Property segment
REVENUES
Net premiums earned	33,559	37,532	103,849	114,011
NET EARNINGS
Net underwriting income	(27,519)	468	(15,852)	13,911
Surety segment
REVENUES
Net premiums earned	30,073	30,444	91,156	90,156
NET EARNINGS
Net underwriting income	$ 9,033	$ 9,877	$ 28,399	$ 23,523