Securities - Schedule of Aging of Past Due Held to Maturity Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities	$ 4,602,232	$ 1,480,416
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities	960,974	999,348
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities	1,144,565	84,934
State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities	$ 2,105,539	1,084,282
30-89 Days Past Due and Accruing [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		7,795
30-89 Days Past Due and Accruing [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		7,795
Total Past Due [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		7,795
Total Past Due [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		7,795
Current [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		991,553
Current [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		84,934
Current [Member] | State and political subdivisions [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Total held to maturity securities		$ 1,076,487