INVESTMENT SECURITIES - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) secuirty	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Available-for-sale securities, amortized cost basis	933,678,000		$ 847,791,000
Securities pledged as collateral	619,000,000		688,200,000
Proceeds from sale of investment securities	7,000,000	$ 263,000,000
Gains from sale of Available-for-sale securities	100,000	$ 300,000
Losses from sale of Available-for-sale securities (less than during the current quarter end)	0
Unamortized premiums	13,900,000		14,100,000
Unaccreted discounts	1,300,000		1,300,000
Owned investment securities	945,300,000
Total unrealized losses on securities	$ 26,900,000
Number of securities, below investment grade | secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 800,000
Weighted average duration of MBS portfolio	5 years 5 months
Mortgage-Backed Securities (MBS)
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0		$ 0