Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 168,837
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	700,497
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	727,661
Available-for-Sale, Amortized Cost Basis, Due after ten years	2,450
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,057,740
Available-for-Sale, Amortized Cost Basis, Total	2,657,185
Available-for-Sale, Estimated Fair Value, Due within one year	170,475
Available-for-Sale, Estimated Fair Value, Due after one year through five years	737,357
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	774,335
Available-for-Sale, Estimated Fair Value, Due after ten years	2,973
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,077,899
Securities available-for-sale, Estimated Fair Value, Total	$ 2,763,039	$ 2,733,899	$ 2,689,330