Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 335,508
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	1,345,049
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,466,782
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,315,847
Available-for-Sale, Amortized Cost Basis, Total	5,463,186	$ 5,463,186	$ 6,152,348	$ 6,546,471	$ 6,546,471
Available-for-Sale, Estimated Fair Value, Due within one year	327,912
Available-for-Sale, Estimated Fair Value, Due after one year through five years	1,214,288
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,034,867
Available-for-Sale, Estimated Fair Value, Due after ten years	1,075,470
Securities available-for-sale, Estimated Fair Value, Total	$ 4,652,537		$ 5,474,359	$ 5,745,443