Premiums Receivable - Summarizes the Activity in the Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums Receivable, Allowance for Doubtful Accounts [Roll Forward]
Balance, beginning of year	$ 5,212	$ 4,852	$ 5,211
Provision for bad debts	1,394	1,646	806
Write-offs	(1,380)	(1,286)	(1,165)
Balance, end of year	$ 5,226	$ 5,212	$ 4,852