v3.26.1
Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 5,716,426 $ 5,724,227
Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,338,149 1,362,212
Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,934,032 1,810,095
State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 3,272,181 3,172,307
Credit Rating, AAA [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 47,599 46,933
Credit Rating, AAA [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 836,298 899,088
Credit Rating, AAA [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 883,897 946,021
Credit Rating, AA [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 50,866 51,390
Credit Rating, AA [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 959,900 777,880
Credit Rating, AA [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,010,766 829,270
Credit Rating, A [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 346,707 379,973
Credit Rating, A [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 122,087 114,845
Credit Rating, A [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 468,794 494,818
Credit Rating, BBB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 820,929 812,061
Credit Rating, BBB [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 13,983 15,824
Credit Rating, BBB [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 834,912 827,885
Credit Rating, BB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 30,143 34,105
Credit Rating, BB [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,764 2,458
Credit Rating, BB [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 31,907 36,563
Credit Rating, B [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 28,447 23,326
Credit Rating, B [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 28,447 23,326
Credit Rating, CCC-C [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 13,458 14,424
Credit Rating, CCC-C [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 13,458 $ 14,424