Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 165,912
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	483,916
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	466,755
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,726
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	2,064,463
Available-for-Sale, Amortized Cost Basis, Total	3,182,772	$ 3,153,568	$ 3,208,412
Available-for-Sale, Estimated Fair Value, Due within one year	167,217
Available-for-Sale, Estimated Fair Value, Due after one year through five years	506,941
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	495,582
Available-for-Sale, Estimated Fair Value, Due after ten years	2,188
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	2,087,564
Securities available-for-sale, Estimated Fair Value, Total	$ 3,259,492	$ 3,158,777	$ 3,197,567