Fair Value of Assets and Liabilities - Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Foreclosed assets	$ 3,533	$ 3,369	$ 2,397
Fair Value, Measurements, Nonrecurring [Member]
Assets
Foreclosed assets	3,533	3,369
Impaired loans, net of allowance	182	473
Significant Unobservable Inputs (Level 3) [Member] \| Fair Value, Measurements, Nonrecurring [Member]
Assets
Foreclosed assets	3,533	3,369
Impaired loans, net of allowance	$ 182	$ 473

Carrying amount as of the balance sheet date of all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings or defeasance; includes real and personal property; equity interests in corporations, partnerships, and joint ventures; and beneficial interests in trusts.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5162-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4437-111522

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