Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available for Sale, Amortized Cost	$ 1,089,011	$ 731,292
Debt Securities Available for Sale, Gross Unrealized Gains	516	2,583
Debt Securities Available for Sale, Gross Unrealized Losses	(33,567)	(5,930)
Debt Securities Available for Sale, Estimated Fair Value	1,055,960	727,945
Debt Securities Held to Maturity, Amortized Cost	459,897	514,844
Debt Securities Held to Maturity, Gross Unrealized Gains	146	5,247
Debt Securities Held to Maturity, Gross Unrealized Losses	(13,756)	(1,926)
Debt Securities Held to Maturity, Estimated Fair Value	446,287	518,165
Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available for Sale, Amortized Cost	666,021	609,695
Debt Securities Available for Sale, Gross Unrealized Gains	163	695
Debt Securities Available for Sale, Gross Unrealized Losses	(27,308)	(5,601)
Debt Securities Available for Sale, Estimated Fair Value	638,876	604,789
Debt Securities Held to Maturity, Amortized Cost	445,309	500,271
Debt Securities Held to Maturity, Gross Unrealized Gains	88	5,101
Debt Securities Held to Maturity, Gross Unrealized Losses	(13,361)	(1,889)
Debt Securities Held to Maturity, Estimated Fair Value	432,036	503,483
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available for Sale, Amortized Cost	129,072	121,597
Debt Securities Available for Sale, Gross Unrealized Gains	107	1,888
Debt Securities Available for Sale, Gross Unrealized Losses	(5,759)	(329)
Debt Securities Available for Sale, Estimated Fair Value	123,420	123,156
Debt Securities Held to Maturity, Amortized Cost	14,588	14,573
Debt Securities Held to Maturity, Gross Unrealized Gains	58	146
Debt Securities Held to Maturity, Gross Unrealized Losses	(395)	(37)
Debt Securities Held to Maturity, Estimated Fair Value	14,251	$ 14,682
Corporate Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available for Sale, Amortized Cost	4,368
Debt Securities Available for Sale, Gross Unrealized Gains	65
Debt Securities Available for Sale, Gross Unrealized Losses	(2)
Debt Securities Available for Sale, Estimated Fair Value	4,431
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities Available for Sale, Amortized Cost	289,550
Debt Securities Available for Sale, Gross Unrealized Gains	181
Debt Securities Available for Sale, Gross Unrealized Losses	(498)
Debt Securities Available for Sale, Estimated Fair Value	$ 289,233

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

No definition available.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (c)
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- Definition

Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

+ References

No definition available.

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
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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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