Subsequent Event - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	Jan. 12, 2018 USD ($)
First Bank & Trust Company [Member]
Subsequent Event [Line Items]
Date of acquisition	Jan. 12, 2018
Total assets acquired	$ 287
Loans	248
Deposits	250
Equity capital	$ 36
First Bank of Chandler [Member]
Subsequent Event [Line Items]
Date of acquisition	Jan. 12, 2018
Total assets acquired	$ 86
Loans	72
Deposits	75
Equity capital	$ 11

Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e845-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e848-128460

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration