Loss and Loss Expense Reserves (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves at the beginning of the year	$ 290,092	$ 218,629	$ 203,253
Provision for losses and loss expenses:
Claims occurring during the current year	147,963	225,251	154,775
Claims occurring during prior years	(9,875)	(9,696)	(8,823)
Total incurred	138,088	215,555	145,952
Loss and loss expense payments:
Claims occurring during the current year	44,942	71,699	56,394
Claims occurring during prior years	94,002	72,393	74,182
Total paid	138,944	144,092	130,576
Reserves at the end of the year	289,236	290,092	218,629
Reinsurance recoverable on unpaid losses at the end of the year	166,218	131,464	125,891
Reserves, gross of reinsurance recoverable, at the end of the year	455,454	421,556	344,520
Savings developed in settlements of claims with comparative developments for previous calendar years	9,875
Period of comparative developments (in years)	2 years
Major Components of the Development of (Savings) deficiency [Abstract]
Property and casualty insurance	(7,111)	(7,417)	(6,567)
Reinsurance	(2,764)	(2,279)	(2,256)
Total	(9,875)	(9,696)	(8,823)
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	6,118	6,642
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	$ 7,839	$ 7,158