Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 465,744	$ 470,825
Gross Unrealized Gains	594	911
Gross Unrealized Losses	(5,475)	(4,033)
Estimated Fair Value	460,863	467,703
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	324,661	314,905
Gross Unrealized Gains	108
Gross Unrealized Losses	(4,574)	(2,103)
Estimated Fair Value	320,195	312,802
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,649	89,098
Gross Unrealized Gains	2	82
Gross Unrealized Losses	(222)	(329)
Estimated Fair Value	84,429	88,851
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,828	18,358
Gross Unrealized Gains	137	204
Gross Unrealized Losses	(559)	(586)
Estimated Fair Value	16,406	17,976
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,606	41,937
Gross Unrealized Gains	347	554
Gross Unrealized Losses	(120)	(121)
Estimated Fair Value	$ 39,833	42,370
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,527
Gross Unrealized Gains		71
Gross Unrealized Losses		(894)
Estimated Fair Value		$ 5,704

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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