Loss and Loss Expense Reserves, Incurred Paid Claims Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2019		Dec. 31, 2018
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 567,630		$ 468,954
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	398,305		375,935
Unallocated claims adjustment expenses	22,370		20,450
Total gross liability for unpaid claims and claims adjustment expense	988,305		865,339
Workers' Compensation [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	154,409		131,996
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	182,908		172,869
Physical Damage [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	11,676	[1]	13,329
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	655		1,851
Commercial Liability [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	355,148		268,523
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	209,152		194,483
Professional Liability Assumed [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	16,079		21,390
Other Short-duration Insurance Lines [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	30,318		33,716
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	$ 5,590		$ 6,732

[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.