Allowance for Loan Losses and Credit Quality Information - Summary of Balance of TDRs (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Modifications [Line Items]
Balance of TDRs	$ 29,688	$ 22,787
Performing
Financing Receivable, Modifications [Line Items]
Balance of TDRs	20,061	14,336
Nonperforming
Financing Receivable, Modifications [Line Items]
Balance of TDRs	$ 9,627	$ 8,451

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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