INVESTMENT SECURITIES - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, amortized cost within one year	$ 0	$ 0
Available-for-sale securities, amortized cost after one year but within five years	19,963	20,051
Available-for-sale securities, amortized cost after five years but within ten years	180,927	179,812
Available-for-sale securities, amortized cost after ten years	732,788	647,928
Available-for-sale securities, amortized cost total	933,678	847,791
Available-for-sale securities, fair value within one year	0	0
Available-for-sale securities, fair value after one year but within five years	19,389	19,825
Available-for-sale securities, fair value after five years but within ten years	173,171	175,583
Available-for-sale securities, fair value after ten years	715,258	642,091
Available-for-sale securities, fair value total	907,818	837,499
Held-to-maturity securities, amortized cost within one year	160	322
Held-to-maturity securities, amortized cost after one year but within five years	6,867	5,895
Held-to-maturity securities, amortized cost after five years but within ten years	20,042	18,751
Held-to-maturity securities, amortized cost after ten years	132,603	136,218
Amortized Cost	159,672	161,186
Held-to-maturity securities, fair value within one year	160	320
Held-to-maturity securities, fair value after one year but within five years	6,850	5,894
Held-to-maturity securities, after five years but within ten years	19,930	18,873
Held-to-maturity securities, after ten years	132,224	137,766
Held-to-maturity securities, fair value total	$ 159,164	$ 162,853