Loss and Loss Expense Reserves, Activity in Reserves for Losses and Major Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 680,274	$ 576,330	$ 513,596
Reinsurance recoverable on unpaid losses at the beginning of the year	308,143	251,563	211,843
Reserves at the beginning of the year	372,131	324,767	301,753
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	329,078	228,303	172,645
Claims occurring during prior years	16,786	19,215	13,836
Total incurred losses and loss expenses	345,864	247,518	186,481
Loss and loss expense payments [Abstract]
Claims occurring during the current year	84,738	67,234	54,239
Claims occurring during prior years	143,853	132,920	109,228
Total paid	228,591	200,154	163,467
Reserves at the end of the year	489,404	372,131	324,767
Reinsurance recoverable on unpaid losses at the end of the year	375,935	308,143	251,563
Reserves, gross of reinsurance recoverable, at the end of the year	865,339	680,274	$ 576,330
Salvage and Subrogation Recoveries [Abstract]
Estimated salvage and subrogation recoverable	$ 7,545	$ 7,559