SCHEDULE III-SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary insurance information
Deferred policy acquisition costs	$ 85,044	$ 84,934	$ 77,716
Unpaid losses and settlement expenses, gross	1,574,352	1,461,348	1,271,503
Unearned premiums, gross	540,213	496,505	451,449
Net premiums earned	839,111	791,366	737,937
Incurred losses and settlement expenses
Current accident year	488,700	478,143	440,452
Prior accident years	(75,284)	(49,950)	(38,868)
Policy acquisition costs	288,697	267,738	252,515
Other operating expenses	69,430	53,803	56,994
Net premiums written	860,337	823,175	749,854
Casualty segment
Supplementary insurance information
Deferred policy acquisition costs	47,805	50,040	44,358
Unpaid losses and settlement expenses, gross	1,435,619	1,283,204	1,127,787
Unearned premiums, gross	354,118	330,836	296,751
Net premiums earned	558,458	523,472	478,603
Incurred losses and settlement expenses
Current accident year	392,653	363,015	323,141
Prior accident years	(62,497)	(33,252)	(17,462)
Policy acquisition costs	166,499	151,007	136,135
Other operating expenses	41,202	31,562	32,885
Net premiums written	564,979	547,177	494,649
Property segment
Supplementary insurance information
Deferred policy acquisition costs	17,057	14,090	13,029
Unpaid losses and settlement expenses, gross	100,000	134,822	107,304
Unearned premiums, gross	116,624	93,032	84,010
Net premiums earned	164,022	149,261	138,346
Incurred losses and settlement expenses
Current accident year	78,075	94,635	97,161
Prior accident years	(4,461)	(10,813)	(12,134)
Policy acquisition costs	55,986	51,830	51,070
Other operating expenses	16,279	12,725	14,108
Net premiums written	181,974	155,601	137,031
Surety segment
Supplementary insurance information
Deferred policy acquisition costs	20,182	20,804	20,329
Unpaid losses and settlement expenses, gross	38,733	43,322	36,412
Unearned premiums, gross	69,471	72,637	70,688
Net premiums earned	116,631	118,633	120,988
Incurred losses and settlement expenses
Current accident year	17,972	20,493	20,150
Prior accident years	(8,326)	(5,885)	(9,272)
Policy acquisition costs	66,212	64,901	65,310
Other operating expenses	11,949	9,516	10,001
Net premiums written	$ 113,384	$ 120,397	$ 118,174