Securities - Summary of Amortized Cost and Estimated Fair Values of Debt Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 723,117	$ 773,574
Gross Unrealized Gains	3,290	1,499
Gross Unrealized Losses	(2,752)	(4,369)
Estimated Fair Value	723,655	770,704
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	651,860	699,882
Gross Unrealized Gains	2,847	1,108
Gross Unrealized Losses	(1,957)	(3,524)
Estimated Fair Value	652,750	697,466
U.S. Federal Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,106	30,079
Gross Unrealized Losses	(152)	(160)
Estimated Fair Value	27,954	29,919
Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,162	16,367
Gross Unrealized Gains	126	114
Gross Unrealized Losses	(576)	(573)
Estimated Fair Value	15,712	15,908
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26,989	27,246
Gross Unrealized Gains	317	277
Gross Unrealized Losses	(67)	(112)
Estimated Fair Value	$ 27,239	$ 27,411

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
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