Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 988,305	$ 865,339
Reinsurance recoverable on unpaid losses at the beginning of the year	398,305	375,935
Reserves at the beginning of the year	590,000	489,404
Provision for losses and loss expenses [Abstract]
Claims occurring during the current period	234,718	263,925
Claims occurring during prior periods	(5)	(1,589)
Total incurred	234,713	262,336
Loss and loss expense payments [Abstract]
Claims occurring during the current period	47,852	53,836
Claims occurring during prior periods	136,231	133,196
Total paid	184,083	187,032
Reserves at the end of the period	640,630	564,708
Reinsurance recoverable on unpaid losses at the end of the period	413,827	395,987
Reserves, gross of reinsurance recoverable, at the end of the period	$ 1,054,457	$ 960,695