Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 1,042,670	$ 1,014,614
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	953,612	925,289
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	89,058	89,325
Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	388,555	396,522
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	332,322	340,290
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	56,233	56,232
Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	629,082	591,879
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	596,257	558,786
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	32,825	33,093
Credit Rating, BB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	17,079	18,078
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	17,079	18,078
Credit Rating, CCC-C [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	7,954	8,135
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 7,954	$ 8,135