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Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 5,654,232	$ 5,378,912
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,378,720	1,093,659
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,663,555	1,645,788
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	3,042,275	2,739,447
Credit Rating, AAA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	48,372
Credit Rating, AAA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	774,751	759,798
Credit Rating, AAA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	823,123	759,798
Credit Rating, AA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	51,835
Credit Rating, AA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	754,910	761,706
Credit Rating, AA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	806,745	761,706
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	401,250	424,690
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	110,326	99,127
Credit Rating, A [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	511,576	523,817
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	797,970	610,351
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	22,895	24,509
Credit Rating, BBB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	820,865	634,860
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	41,021	36,628
Credit Rating, BB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	673	648
Credit Rating, BB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	41,694	37,276
Credit Rating, B [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	23,551
Credit Rating, B [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	23,551
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	14,721	21,990
Credit Rating, CCC-C [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 14,721	$ 21,990

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

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- Definition

Amount, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
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-Section 50
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-Publisher FASB
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-14

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