Ceded Premiums Earned by type (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Premiums Earned by type [Line Items]
Ceded Premiums Written	$ (447,329)	$ (262,340)	$ (183,808)
Increase (Decrease) in Ceded Unearned Premiums	46,796	52,442	15,551
Ceded Premiums Earned	(400,533)	(209,898)	(168,257)
Excess of Loss Product Line
Ceded Premiums Earned by type [Line Items]
Ceded Premiums Written	(419,668)	(235,236)	(163,106)
Equipment and Identity Theft Product Line
Ceded Premiums Earned by type [Line Items]
Ceded Premiums Written	(9,576)	(8,313)	(6,169)
Flood Product Line
Ceded Premiums Earned by type [Line Items]
Ceded Premiums Written	(18,085)	(16,395)	(14,533)
Novation of Auto Policies
Ceded Premiums Earned by type [Line Items]
Ceded Premiums Written	$ 0	$ (2,396)	$ 0