Loss and Loss Expense Reserves, Activity in Reserves for Losses and Major Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 576,330	$ 513,596	$ 506,102
Reinsurance recoverable on unpaid losses at the beginning of the year	251,563	211,843	210,519
Reserves at the beginning of the year	324,767	301,753	295,583
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	228,303	172,645	165,812
Claims occurring during prior years	19,215	13,836	(10,062)
Total incurred losses and loss expenses	247,518	186,481	155,750
Loss and loss expense payments [Abstract]
Claims occurring during the current year	67,234	54,239	56,710
Claims occurring during prior years	132,920	109,228	92,870
Total paid	200,154	163,467	149,580
Reserves at the end of the year	372,131	324,767	301,753
Reinsurance recoverable on unpaid losses at the end of the year	308,143	251,563	211,843
Liability for Unpaid Claims and Claims Adjustment Expense	680,274	576,330	$ 513,596
Salvage and Subrogation Recoveries [Abstract]
Estimated salvage and subrogation recoverable	$ 7,559	$ 4,151