Loss and Loss Expense Reserves (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 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	151,462	103,666
Claims occurring during prior periods	(1,675)	16,687
Total incurred	149,787	120,353
Loss and loss expense payments [Abstract]
Claims occurring during the current period	28,903	23,548
Claims occurring during prior periods	88,965	84,628
Total paid	117,868	108,176
Reserves at the end of the period	404,050	336,944
Reinsurance recoverable on unpaid losses at the end of the period	312,231	290,781
Reserves, gross of reinsurance recoverable, at the end of the period	$ 716,281	$ 627,725