Loss and Loss Expense Reserves, Activity in Reserves for Losses and Major Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves at the beginning of the year	$ 301,753	$ 295,583	$ 288,088
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	172,645	165,812	169,950
Claims occurring during prior years	13,836	(10,062)	(10,354)
Total incurred	186,481	155,750	159,596
Loss and loss expense payments [Abstract]
Claims occurring during the current year	54,239	56,710	59,826
Claims occurring during prior years	109,228	92,870	92,275
Total paid	163,467	149,580	152,101
Reserves at the end of the year	324,767	301,753	295,583
Reinsurance recoverable on unpaid losses at the end of the year	251,563	211,843	210,519
Reserves, gross of reinsurance recoverable, at the end of the year	576,330	513,596	506,102
Savings developed in settlements of claims with comparative developments for previous calendar years	$ 13,836
Period of comparative developments	2 years
Major Components of the Development of (Savings) deficiency [Abstract]
Property and casualty insurance	$ 5,441	(10,289)	(5,423)
Reinsurance	8,395	227	(4,931)
Totals	13,836	(10,062)	$ (10,354)
Salvage and Subrogation Recoveries [Abstract]
Estimated salvage and subrogation recoverable	$ 4,151	$ 4,674