Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 1,089,669	$ 988,305
Reinsurance recoverable on unpaid losses at the beginning of the year	424,368	398,305
Reserves at the beginning of the year, net of reinsurance	665,301	590,000
Provision for losses and loss expenses, net [Abstract]
Claims occurring during the current period	83,143	81,839
Claims occurring during prior periods	(825)	(8)
Total incurred	82,318	81,831
Loss and loss expense payments, net [Abstract]
Claims occurring during the current period	8,082	8,169
Claims occurring during prior periods	52,847	61,812
Total paid	60,929	69,981
Reserves at the end of the period, net of reinsurance	686,690	601,850
Reinsurance recoverable on unpaid losses at the end of the period	421,442	399,749
Reserves, gross of reinsurance recoverable, at the end of the period	$ 1,108,132	$ 1,001,599