Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 134,596
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	751,592
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	739,704
Available-for-Sale, Amortized Cost Basis, Due after ten years	3,781
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,025,569
Available-for-Sale, Amortized Cost Basis, Total	2,655,242
Available-for-Sale, Estimated Fair Value, Due within one year	135,906
Available-for-Sale, Estimated Fair Value, Due after one year through five years	786,916
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	767,254
Available-for-Sale, Estimated Fair Value, Due after ten years	4,498
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,042,779
Securities available-for-sale, Estimated Fair Value, Total	$ 2,737,353	$ 2,415,856	$ 2,253,762