Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

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-Section 50
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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25351-111560

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Investment Securities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investment [Line Items]
Other than temporary impairments			$ 0	$ 0
Securities, carrying value	$ 458,800,000		458,800,000		$ 400,400,000
Equity securities, at fair value	16,038,000		16,038,000		18,089,000
Cumulative adjustment for adoption of ASU 2016-01					0
Loss on equity securities	439,000	$ 0	384,000	$ 0
Investment in government guaranteed loans, mortgage-backed securities, small business loans	9,500,000		9,500,000
Investment in community development loans	3,500,000		$ 3,500,000
Redemption of funds			60 days
Unfunded commitments	0		$ 0
Other financial institutions
Investment [Line Items]
Equity securities, at fair value	3,000,000		3,000,000
Community reinvestment funds
Investment [Line Items]
Equity securities, at fair value	$ 13,000,000		$ 13,000,000
Retained Earnings
Investment [Line Items]
Cumulative adjustment for adoption of ASU 2016-01					$ 2,043,000