Loss and Loss Expense Reserves, Incurred Paid Claims Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Net Outstanding Liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 308,278
Reinsurance Recoverable on Unpaid Claims [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	251,563
Unallocated claims adjustment expenses	16,489
Liability for Claims and Claims Adjustment Expense	576,330	$ 513,596
Workers' Compensation [Member]
Net Outstanding Liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	101,883
Reinsurance Recoverable on Unpaid Claims [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	153,847
Commercial Liability [Member]
Net Outstanding Liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	133,139
Reinsurance Recoverable on Unpaid Claims [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	85,549
Professional Liability Assumed [Member]
Net Outstanding Liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	34,700
Reinsurance Recoverable on Unpaid Claims [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	0
Other Short-duration Insurance Lines [Member]
Net Outstanding Liabilities [Abstract]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	38,556
Reinsurance Recoverable on Unpaid Claims [Abstract]
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	$ 12,167