Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 297,211
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	2,493,104
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	2,506,074
Available-for-Sale, Amortized Cost Basis, Due after ten years	108,800
Available-for-Sale, Amortized Cost Basis, Total	5,405,189	$ 4,177,179	$ 3,927,254
Available-for-Sale, Estimated Fair Value, Due within one year	301,535
Available-for-Sale, Estimated Fair Value, Due after one year through five years	2,586,178
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	2,581,554
Available-for-Sale, Estimated Fair Value, Due after ten years	108,781
Securities available-for-sale, Estimated Fair Value, Total	$ 5,578,048	$ 4,393,029	$ 4,118,863