Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 290,536
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	2,355,100
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,145,293
Available-for-Sale, Amortized Cost Basis, Due after ten years	170,509
Available-for-Sale, Amortized Cost Basis, Total	4,961,438	$ 4,177,179	$ 3,950,510
Available-for-Sale, Estimated Fair Value, Due within one year	295,145
Available-for-Sale, Estimated Fair Value, Due after one year through five years	2,456,956
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,188,324
Available-for-Sale, Estimated Fair Value, Due after ten years	169,206
Securities available-for-sale, Estimated Fair Value, Total	$ 5,109,631	$ 4,393,029	$ 4,107,069