Servicing - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Transfers and Servicing [Abstract]
Loans serviced for others unpaid principal amount	$ 288.6	$ 280.4
Capitalized service right assets included in other assets	2.3	2.2
Fair market value of the capitalized servicing rights	$ 3.1	$ 2.5
Prepayment rate	10.40%	14.30%
Discount yield	6.07%	5.38%

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122596-111746

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Balance Type:	na
Period Type:	duration