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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The number of securities included in the investment securities portfolio.

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- Definition

The number of trust preferred securities included in the investment securities portfolio.

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of debt and equity financial instruments classified as either held-to-maturity or available-for-sale.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale 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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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Amount of unrealized holding loss before gain recognized in the income statement for investments in trading securities.

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Investment Securities - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
Investment Securities - Additional Information (Detail)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment [Line Items]
Trading securities that consist of mutual funds	$ 5,828,000	$ 5,351,000	$ 5,828,000	$ 5,351,000	$ 5,584,000
Held-to-Maturity, Unrecognized Losses	$ 23,993,000		23,993,000		8,000
OTTI losses on securities			$ 0	0
Number of securities in security portfolio \| Security	1,600		1,600
Number of securities in unrealized loss position \| Security	940		940
Unrealized losses	$ 101,896,000		$ 101,896,000		64,322,000
Proceeds from sale of trust preferred securities	5,327,000	98,038,000	136,648,000	284,315,000
Loss from sale of trust preferred securities	74,000	$ 36,000	1,379,000	79,000
Held-to-maturity Securities [Member]
Investment [Line Items]
Unrealized holding loss net of tax					0
Held-to-Maturity, Unrecognized Losses	11,600,000		11,600,000
Former Directors and Executives [Member]
Investment [Line Items]
Trading securities that consist of mutual funds	5,800,000		5,800,000		5,600,000
States and Political Subdivisions [Member]
Investment [Line Items]
Held-to-Maturity, Unrecognized Losses	7,009,000		7,009,000		8,000
Unrealized losses	$ 7,205,000		7,205,000		4,727,000
Pooled Trust Preferred Securities [Member]
Investment [Line Items]
OTTI losses on securities			$ 0	$ 0
Number of pooled trust preferred securities \| Security	2		2
Securities portfolio at fair value	$ 9,100,000		$ 9,100,000
Unrealized losses	4,801,000		4,801,000		$ 8,157,000
Pooled Trust Preferred Securities, Subject To FASB ASC 325-10 [Member]
Investment [Line Items]
Proceeds from sale of trust preferred securities			1,800,000
Loss from sale of trust preferred securities			900,000
Accounting Standards Update 2017-12 [Member]
Investment [Line Items]
Reclassification of securities	324,000,000.0		324,000,000.0
Unrealized holding loss net of tax			10,800,000
Accounting Standards Update 2017-12 [Member] \| States and Political Subdivisions [Member]
Investment [Line Items]
Reclassification of securities	$ 447,000,000.0		$ 447,000,000.0