Schedule III-Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Supplementary insurance information
Deferred policy acquisition costs	$ 103,553	$ 88,425	$ 85,044
Unpaid losses and settlement expenses, gross	2,043,555	1,750,049	1,574,352
Unearned premiums, gross	680,444	586,386	540,213
Net premiums earned	980,903	865,747	839,111
Incurred losses and settlement expenses
Current accident year	582,065	543,937	488,700
Prior accident years	(125,463)	(101,053)	(75,284)
Policy acquisition costs	317,468	286,438	288,697
Other operating expenses	76,907	66,828	69,430
Net premiums written	1,057,533	892,088	860,337
Casualty Segment
Supplementary insurance information
Deferred policy acquisition costs	55,760	48,255	47,805
Unpaid losses and settlement expenses, gross	1,760,469	1,567,544	1,435,619
Unearned premiums, gross	438,248	385,736	354,118
Net premiums earned	633,639	569,521	558,458
Incurred losses and settlement expenses
Current accident year	420,259	397,174	392,653
Prior accident years	(108,632)	(75,075)	(62,497)
Policy acquisition costs	179,354	162,058	166,499
Other operating expenses	47,139	40,937	41,202
Net premiums written	674,709	583,244	564,979
Property segment
Supplementary insurance information
Deferred policy acquisition costs	25,764	19,655	17,057
Unpaid losses and settlement expenses, gross	196,369	150,008	100,000
Unearned premiums, gross	168,209	131,274	116,624
Net premiums earned	231,837	183,720	164,022
Incurred losses and settlement expenses
Current accident year	140,905	124,375	78,075
Prior accident years	(10,981)	(13,019)	(4,461)
Policy acquisition costs	72,008	59,926	55,986
Other operating expenses	18,605	15,620	16,279
Net premiums written	262,816	196,603	181,974
Surety Segment
Supplementary insurance information
Deferred policy acquisition costs	22,029	20,515	20,182
Unpaid losses and settlement expenses, gross	86,717	32,497	38,733
Unearned premiums, gross	73,987	69,376	69,471
Net premiums earned	115,427	112,506	116,631
Incurred losses and settlement expenses
Current accident year	20,901	22,388	17,972
Prior accident years	(5,850)	(12,959)	(8,326)
Policy acquisition costs	66,106	64,454	66,212
Other operating expenses	11,163	10,271	11,949
Net premiums written	$ 120,008	$ 112,241	$ 113,384