Loss and Loss Expense Reserves, Incurred Paid Claims Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 354,282	$ 308,278
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	308,142	251,563
Unallocated claims adjustment expenses	17,850	16,489
Total gross liability for unpaid claims and claims adjustment expense	680,274	576,330
Workers' Compensation [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	113,751	101,883
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	170,394	153,847
Commercial Liability [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	162,581	133,139
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	124,695	85,549
Professional Liability Assumed [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	28,980	34,700
Other Short-duration Insurance Lines [Member]
Net outstanding liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	48,970	38,556
Reinsurance recoverable on unpaid claims [Abstract]
Reinsurance recoverable on unpaid losses	$ 13,053	$ 12,167