Securities - Gross Unrealized Losses and Fair Value of Investment Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	$ 132,271	$ 471,544
Less than 12 months,Unrealized Losses	(4,591)	(12,424)
12 months or more, Fair Value	109,712	546,572
12 months or more, Unrealized Losses	(10,302)	(22,595)
Total Fair Value	241,983	1,018,116
Total Unrealized Losses	(14,893)	(35,019)
State and political subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	132,271	471,544
Less than 12 months,Unrealized Losses	(4,591)	(12,424)
12 months or more, Fair Value	109,712	546,572
12 months or more, Unrealized Losses	(10,302)	(22,595)
Total Fair Value	241,983	1,018,116
Total Unrealized Losses	$ (14,893)	$ (35,019)

Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for 12 months or longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in unrealized loss position.

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

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27290-111563

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

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Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

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