Liability for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss adjustment expenses	$ 432,431	$ 204,694	$ 482,232	$ 140,855
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	266,265	53,319	305,673	18,724
Liability for Unpaid Claims and Claims Adjustment Expense, Net	166,166	151,375	$ 176,559	$ 122,131
Current Year Claims and Claims Adjustment Expense	167,392	152,061
Prior Year Claims and Claims Adjustment Expense	(1,551)	(1,790)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	165,841	150,271
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	83,297	91,732
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	92,937	69,878
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	176,234	161,610
Liability for Unpaid Claims and Claims Adjustment Expense, Reported Claims, Amount	230,926	94,608
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount	$ 201,505	$ 110,086