Mortgage Servicing Assets - Additional Information (Detail) - Mortgage Servicing Assets [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Unpaid principal balances of mortgage and other loans serviced for others	$ 435.5	$ 436.0
Fair value of mortgage servicing rights	$ 3.3	$ 3.3

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

Name:	us-gaap_ServicingAssetAtFairValueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant