Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 1,014,614	$ 1,116,102
Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	396,522
Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	591,879
Credit Rating, BB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	18,078
Credit Rating, CCC-C [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	8,135
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	925,289
Competitive [Member] | Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	340,290
Competitive [Member] | Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	558,786
Competitive [Member] | Credit Rating, BB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	18,078
Competitive [Member] | Credit Rating, CCC-C [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	8,135
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	89,325
Utilities [Member] | Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	56,232
Utilities [Member] | Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 33,093