Reserve for Unpaid Losses - Summary of Reconciliation of Reserve Balances to Liability for Unpaid Loss and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Unpaid Loss and Allocated Loss Adjustment Expense, Net of Reinsurance	$ 212,424
Ceded Unpaid Loss and Allocated Loss Adjustment Expense	393,630	$ 250,507	$ 315,353	$ 589
Unpaid Unallocated Loss Adjustment Expense	7,479
Unpaid losses and loss adjustment expenses	$ 613,533	$ 432,359	$ 470,083	$ 140,137