Schedule of Credit Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Balance at beginning of period	$ 31	$ 31	$ 50
Additions where no OTTI was previously recorded	0	0	0
Additions where an OTTI was previously recorded	0	0	0
Reductions for securities for which the company intends to sell or more likely than not will be required to sell before recovery	0	0	0
Reductions reflecting increases in expected cash flows to be collected	0	0	0
Reductions for securities sold during the period	(18)		(19)
Balance at end of period	$ 13	$ 31	$ 31