Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Liability
Premiums earned	$ 821,249	$ 790,791	$ 772,968
Premiums ceded	(110,347)	(95,315)	(101,510)
Premiums assumed	27,629	37,805	22,691
Net premiums earned	$ 738,531	$ 733,281	$ 694,149
Percentage of amount assumed to net	3.74%	5.16%	3.27%