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	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 126,240
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	725,601
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	657,355
Available-for-Sale, Amortized Cost Basis, Due after ten years	8,878
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,080,604
Available-for-Sale, Amortized Cost Basis, Total	2,598,678
Available-for-Sale, Estimated Fair Value, Due within one year	127,500
Available-for-Sale, Estimated Fair Value, Due after one year through five years	760,706
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	689,777
Available-for-Sale, Estimated Fair Value, Due after ten years	9,742
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,101,605
Securities available-for-sale, Estimated Fair Value	$ 2,689,330	$ 2,415,856	$ 2,163,017