Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	73,899	48,487
Claims occurring during prior periods	(1,601)	112
Total incurred	72,298	48,599
Loss and loss expense payments [Abstract]
Claims occurring during the current period	10,683	9,508
Claims occurring during prior periods	48,655	41,814
Total paid	59,338	51,322
Reserves at the end of the period	385,091	322,044
Reinsurance recoverable on unpaid losses at the end of the period	309,359	256,797
Reserves, gross of reinsurance recoverable, at the end of the period	$ 694,450	$ 578,841