Loss and Loss Expense Reserves, Reconciliation of Net Incurred and Paid Claims Development (Details) - USD ($) $ in Thousands	Dec. 31, 2020		Dec. 31, 2019
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 641,131		$ 567,630
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	424,368		398,305
Unallocated claims adjustment expenses	24,170		22,370
Total gross liability for unpaid claims and claims adjustment expense	1,089,669		988,305
Workers' Compensation [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	168,859		154,409
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	211,421		182,908
Occupational Accident [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	15,039		19,840
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	500		1,979
Physical Damage [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	14,561	[1]	11,676
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	1,054		655
Commercial Liability [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	420,698		355,148
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	209,126		209,152
Professional Liability Assumed [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	11,958		16,079
Other Short-duration Insurance Lines [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	10,016		10,478
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses at the end of the year	$ 2,267		$ 3,611

[1] The majority of physical damage claims settle within a two-year period. The triangles above have been abbreviated to reflect the short-tail nature of this business.