Liability for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Unpaid losses and loss adjustment expenses	$ 544,249	$ 141,102	$ 482,232	$ 140,855
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	399,814	20,152	305,673	18,724
Liability for Unpaid Claims and Claims Adjustment Expense, Net	144,435	120,950	$ 176,559	$ 122,131
Current Year Claims and Claims Adjustment Expense	77,927	63,859
Prior Year Claims and Claims Adjustment Expense	(681)	(526)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	77,246	63,333
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	34,800	21,986
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	74,570	42,528
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	109,370	64,514
Liability for Unpaid Claims and Claims Adjustment Expense, Reported Claims, Amount	214,067	82,987
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount	$ 330,182	$ 58,115