SCHEDULE III-SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary insurance information
Deferred policy acquisition costs	$ 84,934	$ 77,716	$ 73,147
Unpaid losses and settlement expenses, gross	1,461,348	1,271,503	1,139,337
Unearned premiums, gross	496,505	451,449	433,777
Net premiums earned	791,366	737,937	728,608
Incurred losses and settlement expenses
Current accident year	478,143	440,452	391,772
Prior accident years	(49,950)	(38,868)	(41,994)
Policy acquisition costs	267,738	252,515	249,612
Other operating expenses	53,803	56,994	53,093
Net premiums written	823,175	749,854	740,952
Casualty segment
Supplementary insurance information
Deferred policy acquisition costs	50,040	44,358	39,131
Unpaid losses and settlement expenses, gross	1,283,204	1,127,787	1,021,506
Unearned premiums, gross	330,836	296,751	276,096
Net premiums earned	523,472	478,603	454,843
Incurred losses and settlement expenses
Current accident year	363,015	323,141	292,308
Prior accident years	(33,252)	(17,462)	(32,401)
Policy acquisition costs	151,007	136,135	128,566
Other operating expenses	31,562	32,885	30,040
Net premiums written	547,177	494,649	470,082
Property segment
Supplementary insurance information
Deferred policy acquisition costs	14,090	13,029	13,115
Unpaid losses and settlement expenses, gross	134,822	107,304	76,989
Unearned premiums, gross	93,032	84,010	84,425
Net premiums earned	149,261	138,346	152,167
Incurred losses and settlement expenses
Current accident year	94,635	97,161	76,143
Prior accident years	(10,813)	(12,134)	(4,793)
Policy acquisition costs	51,830	51,070	54,167
Other operating expenses	12,725	14,108	13,819
Net premiums written	155,601	137,031	149,170
Surety segment
Supplementary insurance information
Deferred policy acquisition costs	20,804	20,329	20,901
Unpaid losses and settlement expenses, gross	43,322	36,412	40,842
Unearned premiums, gross	72,637	70,688	73,256
Net premiums earned	118,633	120,988	121,598
Incurred losses and settlement expenses
Current accident year	20,493	20,150	23,321
Prior accident years	(5,885)	(9,272)	(4,800)
Policy acquisition costs	64,901	65,310	66,879
Other operating expenses	9,516	10,001	9,234
Net premiums written	$ 120,397	$ 118,174	$ 121,700