Reinsurance (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Premiums Written [Abstract]
Direct										$ 581,070		$ 504,033		$ 395,625
Ceded on direct										(138,102)		(151,348)		(131,166)
Net direct										442,968		352,685		264,459
Assumed										1,430		704		7,379
Ceded on assumed										0		0		(86)
Net assumed										1,430		704		7,293
Net										444,398		353,389		271,752
Premiums Earned [Abstract]
Direct	[1]									562,364		470,158		394,679
Ceded on direct										(131,080)		(145,201)		(129,926)
Net direct										431,284		324,957		264,753
Assumed	[1]									1,596		3,188		11,344
Ceded on assumed										0		0		(86)
Net assumed										1,596		3,188		11,258
Net		$ 118,671	$ 96,807	$ 111,940	$ 105,462	$ 97,075	$ 89,100	$ 67,996	$ 73,974	432,880	[1]	328,145	[1]	276,011	[1]
Net losses and loss expenses incurred reduced by ceded reinsurance recoveries										148,173		128,086		108,656
Net losses and loss (benefit) expense incurred from reinsurance assumed from non-affiliates										(1,300)		5,223		$ 14,746
Components of reinsurance recoverable [Abstract]
Case unpaid losses, net of valuation allowance		163,011				119,615				163,011		119,615
Incurred but not reported unpaid losses and loss expenses		211,805				187,163				211,805		187,163
Paid losses and loss expenses		1,250				2,206				1,250		2,206
Unearned premiums		16,370				9,347				16,370		9,347
Reinsurance recoverable		$ 392,436				$ 318,331				$ 392,436		$ 318,331

[1] Included in Ceded to Other Companies is $0, $0 and $86 for 2018, 2017 and 2016, respectively, relating to retrocessions associated with premiums assumed from other companies. percentage of Amount Assumed to Net above considers the impact of this retrocession.