Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 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	150,366	179,211
Claims occurring during prior periods	(326)	(1,656)
Total incurred	150,040	177,555
Loss and loss expense payments [Abstract]
Claims occurring during the current period	26,246	30,573
Claims occurring during prior periods	103,798	92,822
Total paid	130,044	123,395
Reserves at the end of the period	609,996	543,564
Reinsurance recoverable on unpaid losses at the end of the period	405,364	389,899
Reserves, gross of reinsurance recoverable, at the end of the period	$ 1,015,360	$ 933,463