Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 285,925
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	2,975,374
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,966,491
Available-for-Sale, Amortized Cost Basis, Due after ten years	540,177
Available-for-Sale, Amortized Cost Basis, Total	6,767,967	$ 6,447,510	$ 4,961,438
Available-for-Sale, Estimated Fair Value, Due within one year	287,450
Available-for-Sale, Estimated Fair Value, Due after one year through five years	2,896,943
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,814,749
Available-for-Sale, Estimated Fair Value, Due after ten years	503,353
Securities available-for-sale, Estimated Fair Value, Total	$ 6,502,495	$ 6,573,179	$ 5,109,631