Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 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	88,180	73,899
Claims occurring during prior periods	(1,058)	(1,601)
Total incurred	87,122	72,298
Loss and loss expense payments [Abstract]
Claims occurring during the current period	8,708	10,683
Claims occurring during prior periods	56,747	48,655
Total paid	65,455	59,338
Reserves at the end of the period	511,071	385,091
Reinsurance recoverable on unpaid losses at the end of the period	383,150	309,359
Reserves, gross of reinsurance recoverable, at the end of the period	$ 894,221	$ 694,450