Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross of reinsurance recoverable, at the beginning of the year	$ 680,274	$ 576,330
Reinsurance recoverable on unpaid losses at the beginning of the year	308,143	251,563
Reserves at the beginning of the year	372,131	324,767
Provision for losses and loss expenses [Abstract]
Claims occurring during the current period	229,644	164,546
Claims occurring during prior periods	14,683	16,480
Total incurred	244,327	181,026
Loss and loss expense payments [Abstract]
Claims occurring during the current period	49,510	41,616
Claims occurring during prior periods	123,167	109,616
Total paid	172,677	151,232
Reserves at the end of the period	443,781	354,561
Reinsurance recoverable on unpaid losses at the end of the period	334,056	301,445
Reserves, gross of reinsurance recoverable, at the end of the period	$ 777,837	$ 656,006