Fair Value Measurement - Reconciliation of Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) on Recurring Basis (Detail) - Mortgage Servicing Rights [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Ending Balance	$ 6,687	$ 6,595	$ 7,618
Transfers into (out of) Level 3	0	0	0
Change Included in Earnings	(718)	(2,184)	(701)
Issuances	810	1,161	941
Beginning Balance	$ 6,595	$ 7,618	$ 7,378