Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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	81,839	88,180
Claims occurring during prior periods	(8)	(1,058)
Total incurred	81,831	87,122
Loss and loss expense payments [Abstract]
Claims occurring during the current period	8,169	8,708
Claims occurring during prior periods	61,812	56,747
Total paid	69,981	65,455
Reserves at the end of the period	601,850	511,071
Reinsurance recoverable on unpaid losses at the end of the period	399,749	383,150
Reserves, gross of reinsurance recoverable, at the end of the period	$ 1,001,599	$ 894,221