Liability for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Apr. 03, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss adjustment expenses	$ 528,842	$ 682,161	$ 482,232		$ 140,855
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	332,229	471,498	305,673		18,724
Liability for Unpaid Claims and Claims Adjustment Expense, Net	196,613	210,663	$ 176,559	$ 40,583	$ 122,131
Current Year Claims and Claims Adjustment Expense	290,600	296,217
Prior Year Claims and Claims Adjustment Expense	(4,207)	(2,819)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	286,393	293,398
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	163,589	160,952
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	102,750	84,497
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	266,339	245,449
Liability for Unpaid Claims and Claims Adjustment Expense, Reported Claims, Amount	259,678	289,938
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount	$ 269,164	$ 392,223