Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves at the beginning of the year	$ 324,767	$ 301,753
Provision for losses and loss expenses [Abstract]
Claims occurring during the current year	103,666	83,052
Claims occurring during prior years	16,687	(1,763)
Total incurred	120,353	81,289
Loss and loss expense payments [Abstract]
Claims occurring during the current year	23,548	18,973
Claims occurring during prior years	84,628	56,372
Total paid	108,176	75,345
Reserves at the end of the period	336,944	307,697
Reinsurance recoverable on unpaid losses at the end of the period	290,781	233,651
Reserves, gross of reinsurance recoverable, at the end of the period	627,725	$ 541,348
Savings developed in settlements of claims with comparative developments for previous calendar years	$ 16,687