SCHEDULE III-SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary insurance information
Deferred policy acquisition costs	$ 73,147	$ 69,829	$ 65,123
Unpaid losses and settlement expenses, gross	1,139,337	1,103,785	1,121,040
Unearned premiums, gross	433,777	422,094	401,412
Net premiums earned	728,608	700,161	687,375
Incurred losses and settlement expenses
Current accident year	391,772	364,472	361,451
Prior accident years	(41,994)	(65,427)	(64,842)
Policy acquisition costs	249,612	241,078	229,283
Other operating expenses	53,093	51,480	54,464
Net premiums written	740,952	721,971	703,152
Casualty segment
Supplementary insurance information
Deferred policy acquisition costs	39,131	35,464	31,334
Unpaid losses and settlement expenses, gross	1,021,506	993,717	964,699
Unearned premiums, gross	276,096	260,227	237,117
Net premiums earned	454,843	412,248	382,105
Incurred losses and settlement expenses
Current accident year	292,308	264,068	251,958
Prior accident years	(32,401)	(45,654)	(52,825)
Policy acquisition costs	128,566	119,529	108,747
Other operating expenses	30,040	28,042	28,284
Net premiums written	470,082	435,409	395,853
Property segment
Supplementary insurance information
Deferred policy acquisition costs	13,115	13,332	13,827
Unpaid losses and settlement expenses, gross	76,989	77,584	120,858
Unearned premiums, gross	84,425	88,808	94,092
Net premiums earned	152,167	170,924	197,776
Incurred losses and settlement expenses
Current accident year	76,143	81,699	90,712
Prior accident years	(4,793)	(11,848)	(1,123)
Policy acquisition costs	54,167	57,214	58,646
Other operating expenses	13,819	14,834	16,623
Net premiums written	149,170	166,659	195,580
Surety segment
Supplementary insurance information
Deferred policy acquisition costs	20,901	21,033	19,962
Unpaid losses and settlement expenses, gross	40,842	32,484	35,483
Unearned premiums, gross	73,256	73,059	70,203
Net premiums earned	121,598	116,989	107,494
Incurred losses and settlement expenses
Current accident year	23,321	18,705	18,781
Prior accident years	(4,800)	(7,925)	(10,894)
Policy acquisition costs	66,879	64,335	61,890
Other operating expenses	9,234	8,604	9,557
Net premiums written	$ 121,700	$ 119,903	$ 111,719