Liability for Unpaid Losses and Loss Adjustment Expense - Reconciliation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance Recoverables on Unpaid Losses	$ 305,673	$ 18,724	$ 2,114
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	12,821	6,214
Unpaid losses and loss adjustment expenses	482,232	140,855	$ 76,792	$ 54,436
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	163,738	115,917
Other Short-duration Insurance Product Line
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance Recoverables on Unpaid Losses	8,779	4,501
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	12,384	6,031
Property and Casualty, Commercial Insurance Product Line
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance Recoverables on Unpaid Losses	165,313	0
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	49,810	566
Property and Casualty, Personal Insurance Product Line
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Reinsurance Recoverables on Unpaid Losses	131,581	14,223
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 101,544	$ 109,320