Ceded Reinsurance - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Minimum amount of policyholders' surplus for reinsurance	$ 500,000
Reinsurance rating description	A reinsurer generally must have a rating from A.M. Best and/or S&P of “A” or better, or an equivalent financial strength if not rated, plus at least $500 million in policyholders’ surplus to meet our standards of acceptability.
Reserve for uncollectible reinsurance amount	$ 12,600	$ 12,100
Collateral held	180,664
Ceded Balances Payable
Effects of Reinsurance [Line Items]
Collateral held	136,400
Letter of Credit
Effects of Reinsurance [Line Items]
Collateral held	41,300
Trust Account Balances
Effects of Reinsurance [Line Items]
Collateral held	$ 3,000