Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
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
Reinsurance recoverable on unpaid losses at the beginning of the year	375,935	308,143
Reserves at the beginning of the year	489,404	372,131
Provision for losses and loss expenses [Abstract]
Claims occurring during the current period	263,925	229,644
Claims occurring during prior periods	(1,589)	14,683
Total incurred	262,336	244,327
Loss and loss expense payments [Abstract]
Claims occurring during the current period	53,836	49,510
Claims occurring during prior periods	133,196	123,167
Total paid	187,032	172,677
Reserves at the end of the period	564,708	443,781
Reinsurance recoverable on unpaid losses at the end of the period	395,987	334,056
Reserves, gross of reinsurance recoverable, at the end of the period	$ 960,695	$ 777,837