Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 343,112
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	1,374,546
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,421,721
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,078,138
Available-for-Sale, Amortized Cost Basis, Total	5,217,517	$ 5,243,681	$ 5,879,040
Available-for-Sale, Estimated Fair Value, Due within one year	337,823
Available-for-Sale, Estimated Fair Value, Due after one year through five years	1,288,485
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,126,343
Available-for-Sale, Estimated Fair Value, Due after ten years	905,875
Securities available-for-sale, Estimated Fair Value, Total	$ 4,658,526	$ 4,732,762	$ 5,298,557