Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 187,577
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	552,075
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	448,087
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,708
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,986,797
Available-for-Sale, Amortized Cost Basis, Total	3,176,244	$ 3,042,618	$ 2,825,560
Available-for-Sale, Estimated Fair Value, Due within one year	189,177
Available-for-Sale, Estimated Fair Value, Due after one year through five years	565,059
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	450,670
Available-for-Sale, Estimated Fair Value, Due after ten years	1,951
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,937,510
Securities available-for-sale, Estimated Fair Value, Total	$ 3,144,367	$ 3,087,473	$ 2,885,483