Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 4,602,232	$ 1,480,416
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	960,974	999,348
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,144,565	84,934
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	2,105,539	1,084,282
Credit Rating, AAA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	342,489
Credit Rating, AAA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	342,489
Credit Rating, AA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	690,649
Credit Rating, AA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	690,649
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	379,924	372,696
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	81,855	55,096
Credit Rating, A [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	461,779	427,792
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	560,001	605,104
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	29,572	29,838
Credit Rating, BBB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	589,573	634,942
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	20,219	20,678
Credit Rating, BB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	20,219	20,678
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	830	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	$ 830	$ 870