Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 1,084,009	$ 1,014,614
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	995,444	925,289
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	88,565	89,325
Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	403,031	396,522
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	346,811	340,290
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	56,220	56,232
Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	657,082	591,879
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	624,737	558,786
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	32,345	33,093
Credit Rating, BB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	17,075	18,078
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	17,075	18,078
Credit Rating, CCC-C [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	6,821	8,135
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 6,821	$ 8,135