Mortgage Banking Activities - Valuation Allowance for Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Mortgage Banking [Abstract]
Balance, beginning of period	$ (20)	$ (5)	$ (9)
Impairment charges	(6)	(10)	(26)	$ (15)
Recoveries			9
Balance, end of period	$ (26)	$ (15)	$ (26)	$ (15)