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, 2017	Dec. 31, 2016	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 187,370
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	650,523
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	632,562
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,684
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,353,421
Available-for-Sale, Amortized Cost Basis, Total	2,825,560	$ 2,827,772	$ 2,623,934
Available-for-Sale, Estimated Fair Value, Due within one year	189,136
Available-for-Sale, Estimated Fair Value, Due after one year through five years	679,535
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	656,623
Available-for-Sale, Estimated Fair Value, Due after ten years	2,043
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,358,146
Securities available-for-sale, Estimated Fair Value, Total	$ 2,885,483	$ 2,860,837	$ 2,729,030