Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 5,732,583	$ 5,861,599
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,145,711	1,074,524
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	2,819,300	2,772,922
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,673,589	1,698,398
Credit Rating, AAA [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	757,467
Credit Rating, AAA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities		759,539
Credit Rating, AAA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	757,467	759,539
Credit Rating, AA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	6,588
Credit Rating, AA [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	802,390	824,386
Credit Rating, AA [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	795,802	824,386
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	460,569	435,953
Credit Rating, A [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	551,729	520,246
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	91,160	84,293
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	645,173	618,517
Credit Rating, BBB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	673,727	648,116
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	28,554	29,599
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	14,490	17,120
Credit Rating, BB [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	15,096	17,701
Credit Rating, BB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	606	581
Credit Rating, B [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	16,170
Credit Rating, B [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	16,170
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	2,721	2,934
Credit Rating, CCC-C [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 2,721	$ 2,934