Loss and Loss Expense Reserves, Activity in Reserves for Losses and Major Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 865,339	$ 680,274	$ 576,330
Reinsurance recoverable on unpaid losses at the beginning of the year	375,935	308,143	251,563
Reserves at the beginning of the year	489,404	372,131	324,767
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	349,018	329,078	228,303
Claims occurring during prior years	(550)	16,786	19,215
Total incurred losses and loss expenses	348,468	345,864	247,518
Loss and loss expense payments [Abstract]
Claims occurring during the current year	90,364	84,738	67,234
Claims occurring during prior years	157,508	143,853	132,920
Total paid	247,872	228,591	200,154
Reserves at the end of the year	590,000	489,404	372,131
Reinsurance recoverable on unpaid losses at the end of the year	398,305	375,935	308,143
Reserves, gross of reinsurance recoverable, at the end of the period	988,305	865,339	$ 680,274
Salvage and Subrogation Recoveries [Abstract]
Estimated salvage and subrogation recoverable	$ 4,000	$ 7,545