Securities - Schedule of Amortized Cost Basis by Credit Rating of Held to Maturity Investment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 1,114,160	$ 1,116,102
Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	1,036,844
Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	77,316
Credit Rating, AA [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	8,998
Credit Rating, AA [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	8,998
Credit Rating, A [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	451,431
Credit Rating, A [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	394,082
Credit Rating, A [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	57,349
Credit Rating, BBB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	617,951
Credit Rating, BBB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	597,984
Credit Rating, BBB [Member] | Utilities [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	19,967
Credit Rating, BB [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	22,519
Credit Rating, BB [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	22,519
Credit Rating, B [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	685
Credit Rating, B [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	685
Credit Rating, CCC-C [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	12,576
Credit Rating, CCC-C [Member] | Competitive [Member]
Debt Securities Held To Maturity Credit Quality Indicator [Line Items]
Held to maturity securities	$ 12,576