Loss and Loss Expense Reserves (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves at the beginning of the year	$ 288,088	$ 289,236	$ 290,092
Provision for losses and loss expenses:
Claims occurring during the current year	169,950	156,264	147,963
Claims occurring during prior years	(10,354)	(5,563)	(9,875)
Total incurred	159,596	150,701	138,088
Loss and loss expense payments:
Claims occurring during the current year	59,826	47,908	44,941
Claims occurring during prior years	92,275	103,941	94,003
Total paid	152,101	151,849	138,944
Reserves at the end of the year	295,583	288,088	289,236
Reinsurance recoverable on unpaid losses at the end of the year	210,519	186,382	166,218
Reserves, gross of reinsurance recoverable, at the end of the year	506,102	474,470	455,454
Savings developed in settlements of claims with comparative developments for previous calendar years	10,354
Period of comparative developments (in years)	2 years
Major Components of the Development of (Savings) deficiency [Abstract]
Property and casualty insurance	(5,423)	(1,725)	(7,111)
Reinsurance	(4,931)	(3,838)	(2,764)
Total	(10,354)	(5,563)	(9,875)
Workers' Compensation Discount [Abstract]
Loss reserves on certain permanent total disability workers' compensation reserves discounted to present value at pre-tax rates (in hundredths)	3.50%
Amount of reduction in loss reserves	3,129	5,885
Salvage and Subrogation Recoveries [Abstract]
Salvage and Subrogation Recoveries, Description	Loss reserves have been reduced by estimated salvage and subrogation recoverable.
Estimated salvage and subrogation recoverable	$ 7,158	$ 8,115