Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 446,674	$ 465,315
Gross Unrealized Gains	2,098	2,324
Gross Unrealized Losses	(1,831)	(2,171)
Estimated Fair Value	446,941	465,468
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	284,874	268,763
Gross Unrealized Gains	693	700
Gross Unrealized Losses	(293)	(920)
Estimated Fair Value	285,274	268,543
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	94,150	129,674
Gross Unrealized Gains	212	373
Gross Unrealized Losses	(251)	(405)
Estimated Fair Value	94,111	129,642
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,621	19,949
Gross Unrealized Gains	231	290
Gross Unrealized Losses	(577)	(567)
Estimated Fair Value	18,275	19,672
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,678	40,335
Gross Unrealized Gains	773	836
Gross Unrealized Losses	(25)	(129)
Estimated Fair Value	43,426	41,042
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,351	6,594
Gross Unrealized Gains	189	125
Gross Unrealized Losses	(685)	(150)
Estimated Fair Value	$ 5,855	$ 6,569

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.

+ References

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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-SubTopic 10
-Section S99
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