Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Activity in the reserve for losses and loss expenses [Roll Forward]
Reserves, gross reinsurance recoverable, at beginning of the year	$ 576,330	$ 513,596
Reinsurance recoverable on unpaid losses at the beginning of the year	251,563	211,843
Reserves at the beginning of the year	324,767	301,753
Provision for losses and loss expenses [Abstract]
Claims occurring during the current period	164,546	129,404
Claims occurring during prior periods	16,480	8,712
Total incurred	181,026	138,116
Loss and loss expense payments [Abstract]
Claims occurring during the current period	41,616	30,932
Claims occurring during prior periods	109,616	81,427
Total paid	151,232	112,359
Reserves at the end of the period	354,561	327,510
Reinsurance recoverable on unpaid losses at the end of the period	301,445	238,049
Reserves, gross of reinsurance recoverable, at the end of the period	656,006	565,559
Savings developed in settlements of claims with comparative developments for previous calendar years	$ 16,480	$ 8,712