Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated earned premiums [Abstract]
Gross amount	$ 384,993	$ 382,300	$ 366,752
Ceded to other companies	437,881	426,946	407,531
Assumed from other companies	660,046	637,054	611,045
Net amount	$ 607,158	$ 592,408	$ 570,266
Percentage of amount assumed to net	108.70%	107.50%	107.20%