Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 304,065
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	1,709,307
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,143,482
Available-for-Sale, Amortized Cost Basis, Due after ten years	975,597
Available-for-Sale, Amortized Cost Basis, Total	5,132,451	$ 5,243,681	$ 5,687,352
Available-for-Sale, Estimated Fair Value, Due within one year	299,230
Available-for-Sale, Estimated Fair Value, Due after one year through five years	1,576,132
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	1,856,004
Available-for-Sale, Estimated Fair Value, Due after ten years	841,658
Available-for-Sale, Estimated Fair Value, Total	$ 4,573,024	$ 4,732,762	$ 5,066,262