Schedule of Credit Losses Recognized in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Balance at beginning of period	$ 31	$ 31	$ 31	$ 31
Additions where no OTTI was previously recorded	0	0	0	0
Additions where an OTTI was previously recorded	0	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	0
Reductions reflecting increases in expected cash flows to be collected	0	0	0	0
Reductions for securities sold during the period	(15)		(15)
Balance at end of period	$ 16	$ 31	$ 16	$ 31