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, 2016	Dec. 31, 2015	Jun. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 180,342
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	669,211
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	762,452
Available-for-Sale, Amortized Cost Basis, Due after ten years	9,919
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,049,113
Available-for-Sale, Amortized Cost Basis, Total	2,671,037	$ 2,654,885	$ 2,669,891
Available-for-Sale, Estimated Fair Value, Due within one year	182,079
Available-for-Sale, Estimated Fair Value, Due after one year through five years	705,585
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	824,643
Available-for-Sale, Estimated Fair Value, Due after ten years	10,578
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,072,471
Securities available-for-sale, Estimated Fair Value, Total	$ 2,795,356