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Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($)
$ in Thousands
Jun. 30, 2022
Dec. 31, 2021
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 5,709,541 $ 1,480,416
Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,040,601 999,348
Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 1,457,093  
State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 2,497,694 1,084,282
Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities   84,934
Credit Rating, AAA [Member] | Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 576,670  
Credit Rating, AAA [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 576,670  
Credit Rating, AA [Member] | Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 769,033  
Credit Rating, AA [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 769,033  
Credit Rating, A [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 392,343 372,696
Credit Rating, A [Member] | Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 84,104  
Credit Rating, A [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 476,447 427,792
Credit Rating, A [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities   55,096
Credit Rating, BBB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 628,494 605,104
Credit Rating, BBB [Member] | Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 26,722  
Credit Rating, BBB [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 655,216 634,942
Credit Rating, BBB [Member] | Utilities [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities   29,838
Credit Rating, BB [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 18,975 20,678
Credit Rating, BB [Member] | Utilities and General Obligation [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 564  
Credit Rating, BB [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 19,539 20,678
Credit Rating, CCC-C [Member] | Competitive [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities 789 870
Credit Rating, CCC-C [Member] | State and political subdivisions [Member]    
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]    
Held to maturity securities $ 789 $ 870