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, 2016	Dec. 31, 2015	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 195,637
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	675,403
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	740,541
Available-for-Sale, Amortized Cost Basis, Due after ten years	2,044
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,010,309
Available-for-Sale, Amortized Cost Basis, Total	2,623,934	$ 2,654,885	$ 2,655,242
Available-for-Sale, Estimated Fair Value, Due within one year	197,334
Available-for-Sale, Estimated Fair Value, Due after one year through five years	710,096
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	788,744
Available-for-Sale, Estimated Fair Value, Due after ten years	2,610
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,030,246
Securities available-for-sale, Estimated Fair Value, Total	$ 2,729,030	$ 2,733,899	$ 2,737,353