Ceded Reinsurance - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Effects Of Reinsurance [Line Items]
Reserve for uncollectible reinsurance amount	$ 12,400	$ 12,100
Collateral held	171,616
Ceded Balances Payable
Effects Of Reinsurance [Line Items]
Collateral held	129,300
Letter of Credit
Effects Of Reinsurance [Line Items]
Collateral held	39,700
Trust Account Balances
Effects Of Reinsurance [Line Items]
Collateral held	$ 2,600