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Securities - Shedule of Aging of Past due Held to Maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	$ 5,703,238	$ 5,724,227
Competitive [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	1,343,903	1,362,212
Competitive [Member] | 30-89 Days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	131
Competitive [Member] | Greater than 90 days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	16,470
Competitive [Member] | Total Past Due [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	16,601
Competitive [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	1,327,302
Utilities [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	1,857,775	1,810,095
Utilities [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	1,857,775
State and political subdivisions [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	3,201,678	$ 3,172,307
State and political subdivisions [Member] | 30-89 Days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	131
State and political subdivisions [Member] | Greater than 90 days Past Due and Accruing [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	16,470
State and political subdivisions [Member] | Total Past Due [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	16,601
State and political subdivisions [Member] | Current [Member]
Debt Securities, Held-to-Maturity, Credit Quality Indicator [Line Items]
Held to maturity securities	$ 3,185,077

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- 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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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
-Section 45
-Paragraph 1A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482736/825-10-45-1A

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-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-7A

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-5

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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