Loss and Loss Expense Reserves, Activity in Reserves for Losses and Loss Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 988,305	$ 865,339	$ 680,274
Reinsurance recoverable on unpaid losses at the beginning of the year	398,305	375,935	308,143
Reserves at the beginning of the year	590,000	489,404	372,131
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	319,269	349,018	329,078
Claims occurring during prior years	(311)	(550)	16,786
Total incurred losses and loss expenses	318,958	348,468	345,864
Loss and loss expense payments [Abstract]
Claims occurring during the current year	77,880	90,364	84,738
Claims occurring during prior years	165,777	157,508	143,853
Total paid	243,657	247,872	228,591
Reserves at the end of the year	665,301	590,000	489,404
Reinsurance recoverable on unpaid losses at the end of the year	424,368	398,305	375,935
Reserves, gross of reinsurance recoverable, at the end of the year	1,089,669	988,305	865,339
Salvage and Subrogation Recoveries [Abstract]
Estimated salvage and subrogation recoverable	4,000	4,000
Workers' Compensation [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	1,431	(2,733)	(9,601)
Commercial Auto Liability [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	10,717	8,893	32,342
Physical Damage [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	(1,684)	(4,561)	(1,066)
Professional Liability Assumed [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	189	259	2,343
Occupational Accident [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	(7,467)	688	(4,178)
Other Short-duration Contracts [Member]
Provision for losses and loss expenses [Abstract]
Claims occurring during prior years	$ (3,497)	$ (3,096)	$ (3,054)