Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Written [Abstract]
Direct									$ 504,033	$ 395,625	$ 366,668
Ceded on direct									(151,348)	(131,166)	(128,338)
Net direct									352,685	264,459	238,330
Assumed									704	7,379	16,885
Ceded on assumed									0	(86)	(562)
Net assumed									704	7,293	16,323
Net									353,389	271,752	254,653
Premium Earned [Abstract]
Direct									470,158	394,679	370,499
Ceded on direct									(145,201)	(129,926)	(128,135)
Net direct									324,957	264,753	242,364
Assumed									3,188	11,344	21,533
Ceded on assumed									0	(86)	(562)
Net assumed									3,188	11,258	20,971
Net	$ 97,075	$ 89,100	$ 67,996	$ 73,974	$ 69,141	$ 71,235	$ 68,726	$ 66,909	328,145	276,011	263,335
Net losses and loss expenses incurred reduced by ceded reinsurance recoveries									128,086	108,656	75,581
Net losses and loss expense incurred from reinsurance assumed from non-affiliates									5,223	14,746	$ 13,492
Components of reinsurance recoverable [Abstract]
Case unpaid losses, net of valuation allowance	119,615				126,244				119,615	126,244
Incurred but not reported unpaid losses and loss expenses	187,163				123,819				187,163	123,819
Paid losses and loss expenses	2,206				1,760				2,206	1,760
Unearned premiums	9,347				3,201				9,347	3,201
Reinsurance recoverable	$ 318,331				$ 255,024				$ 318,331	$ 255,024