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, 2018	Dec. 31, 2017	Jun. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 183,132
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	609,481
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	478,355
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,701
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,935,743
Available-for-Sale, Amortized Cost Basis, Total	3,208,412	$ 3,042,618	$ 2,905,245
Available-for-Sale, Estimated Fair Value, Due within one year	184,671
Available-for-Sale, Estimated Fair Value, Due after one year through five years	626,252
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	486,921
Available-for-Sale, Estimated Fair Value, Due after ten years	1,981
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,897,742
Securities available-for-sale, Estimated Fair Value, Total	$ 3,197,567	$ 3,087,473	$ 2,964,513