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, 2017	Dec. 31, 2016	Mar. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 195,694
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	671,223
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	793,021
Available-for-Sale, Amortized Cost Basis, Due after ten years	2,064
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	1,313,988
Available-for-Sale, Amortized Cost Basis, Total	2,975,990	$ 2,827,772	$ 2,657,185
Available-for-Sale, Estimated Fair Value, Due within one year	197,227
Available-for-Sale, Estimated Fair Value, Due after one year through five years	697,730
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	805,607
Available-for-Sale, Estimated Fair Value, Due after ten years	2,404
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	1,315,311
Securities available-for-sale, Estimated Fair Value, Total	$ 3,018,279	$ 2,860,837	$ 2,763,039