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, 2017	Dec. 31, 2016	Jun. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 177,217
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	671,763
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	740,378
Available-for-Sale, Amortized Cost Basis, Due after ten years	1,679
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,314,208
Available-for-Sale, Amortized Cost Basis, Total	2,905,245	$ 2,827,772	$ 2,671,037
Available-for-Sale, Estimated Fair Value, Due within one year	178,504
Available-for-Sale, Estimated Fair Value, Due after one year through five years	700,451
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	765,419
Available-for-Sale, Estimated Fair Value, Due after ten years	2,017
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,318,122
Securities available-for-sale, Estimated Fair Value, Total	$ 2,964,513	$ 2,860,837	$ 2,795,356