Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 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	179,211	151,462
Claims occurring during prior periods	(1,656)	(1,675)
Total incurred	177,555	149,787
Loss and loss expense payments [Abstract]
Claims occurring during the current period	30,573	28,903
Claims occurring during prior periods	92,822	88,965
Total paid	123,395	117,868
Reserves at the end of the period	543,564	404,050
Reinsurance recoverable on unpaid losses at the end of the period	389,899	312,231
Reserves, gross of reinsurance recoverable, at the end of the period	$ 933,463	$ 716,281