Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 81,336
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	694,864
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	550,362
Available-for-Sale, Amortized Cost Basis, Due after ten years	6,534
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,005,587
Available-for-Sale, Amortized Cost Basis, Total	2,338,683
Available-for-Sale, Estimated Fair Value, Due within one year	82,078
Available-for-Sale, Estimated Fair Value, Due after one year through five years	726,392
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	580,250
Available-for-Sale, Estimated Fair Value, Due after ten years	7,941
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,019,195
Securities available-for-sale, Estimated Fair Value	$ 2,415,856	$ 2,057,723