Loss and Loss Expense Reserves, Incurred Paid Claims Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2018		Dec. 31, 2017
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 468,954		$ 354,282
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	375,935		308,142
Unallocated claims adjustment expenses	20,450		17,850
Total gross liability for unpaid claims and claims adjustment expense	865,339		680,274
Workers' Compensation [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	131,996		113,751
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	172,869		170,394
Physical Damage [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	13,329	[1]	9,087
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	1,851		51
Commercial Liability [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	268,523		162,581
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	194,483		124,695
Professional Liability Assumed [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	21,390		28,980
Other Short-duration Insurance Lines [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	33,716		39,883
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	$ 6,732		$ 13,002

[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.