Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 128,535
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	625,856
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	1,079,767
Available-for-Sale, Amortized Cost Basis, Due after ten years	16,895
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	2,076,201
Available-for-Sale, Amortized Cost Basis, Total	3,927,254	$ 3,327,805	$ 3,182,772
Available-for-Sale, Estimated Fair Value, Due within one year	129,521
Available-for-Sale, Estimated Fair Value, Due after one year through five years	669,229
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	1,148,866
Available-for-Sale, Estimated Fair Value, Due after ten years	17,354
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	2,153,893
Securities available-for-sale, Estimated Fair Value, Total	$ 4,118,863	$ 3,413,317	$ 3,259,492