Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value Disclosures [Abstract]
Impaired loans	$ 992	$ (238)	$ (6,544)	$ (1,278)
Other real estate owned	(437)	(1,094)	(437)	(1,125)
Total losses	$ 555	$ (1,332)	$ (6,981)	$ (2,403)