Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 151,083
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	621,902
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	971,325
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,745
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	2,204,455
Available-for-Sale, Amortized Cost Basis, Total	3,950,510	$ 3,327,805	$ 3,172,535
Available-for-Sale, Estimated Fair Value, Due within one year	152,516
Available-for-Sale, Estimated Fair Value, Due after one year through five years	654,116
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	1,013,634
Available-for-Sale, Estimated Fair Value, Due after ten years	2,269
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	2,284,534
Securities available-for-sale, Estimated Fair Value, Total	$ 4,107,069	$ 3,413,317	$ 3,212,812