Tax Cuts and Jobs Act of 2017, reclassification from AOCI to retained earnings tax effect.

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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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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of cumulative effect of the change in accounting principle or new accounting pronouncement on retained earnings or other components of equity or net assets in the statement of financial position as of the beginning of the earliest period presented.

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Represents the quantification of the effect of adopting the new accounting standard or change in accounting principle expected by the entity to have a significant effect on the entity's financial statements.

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Line items represent financial 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.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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Recent Accounting Pronouncements - Additional Information (Detail) - USD ($)		9 Months Ended
	Jan. 02, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Modified retrospective basis through a cumulative-effect adjustment to retained earnings		$ 1,000,000.0
Retained earnings		503,147,000	$ 413,130,000	$ 451,461,000
Unrealized holding gains on available-for-sale portfolio		5,736,000	43,648,000
Reclassification of certain tax effects related to the Tax Cuts and Jobs Act of 2017 (Note 3)	$ 10,800,000
States and Political Subdivisions [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unrealized holding gains on available-for-sale portfolio		5,691,000	$ 43,495,000
Accounting Standards Update 2017-12 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		3,200,000
Reclassification of securities		324,000,000.0
Unrealized holding gains on available-for-sale portfolio		26,100,000
Accounting Standards Update 2017-12 [Member] \| States and Political Subdivisions [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of securities		447,000,000.0
Minimum [Member] \| Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated increase in assets and liabilities		125,000,000
Maximum [Member] \| Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated increase in assets and liabilities		$ 140,000,000