Loan Servicing - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2015
Receivables [Abstract]
Unpaid principal balances of loans serviced for others totaled	$ 1,310,000	$ 1,301,000
Aggregate fair value of capitalized mortgage servicing rights	12,800	12,100	$ 10,800	$ 7,600
Mortgage servicing rights, net	11,602	11,174	8,874
Bank recorded additional impairment	$ 80	$ 110	$ 59

The adjustment to the carrying value of the rights retained or purchased to service mortgages. These adjustments are made when the estimate of the fair value is changed. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (a)(6),(7)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122739-111746

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration