Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 465,315	$ 541,670
Gross Unrealized Gains	2,324	3,424
Gross Unrealized Losses	(2,171)	(934)
Estimated Fair Value	465,468	544,160
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	268,763	328,965
Gross Unrealized Gains	700	776
Gross Unrealized Losses	(920)	(45)
Estimated Fair Value	268,543	329,696
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129,674	131,522
Gross Unrealized Gains	373	527
Gross Unrealized Losses	(405)	(153)
Estimated Fair Value	129,642	131,896
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,949	21,973
Gross Unrealized Gains	290	425
Gross Unrealized Losses	(567)	(543)
Estimated Fair Value	19,672	21,855
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,335	49,521
Gross Unrealized Gains	836	1,447
Gross Unrealized Losses	(129)	(48)
Estimated Fair Value	41,042	50,920
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,594	9,689
Gross Unrealized Gains	125	249
Gross Unrealized Losses	(150)	(145)
Estimated Fair Value	$ 6,569	$ 9,793

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)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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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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