INVESTMENT SECURITIES - Additional Information (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) secuirty	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0
Securities pledged as collateral	1,000,000,000		$ 914,500,000
Sale of investment securities available for sale	583,852,000	$ 7,012,000
Remaining securities	5,100,000
Debt securities, available-for-sale, realized gain (less than)	100,000	100,000
Losses from sale of available-for-sale securities (less than during the current quarter end)	0	$ 0
Unamortized premiums	14,200,000		12,700,000
Unaccreted discounts	3,300,000		2,500,000
Owned investment securities	764,200,000
Total unrealized losses on securities	$ 9,400,000
Number of securities, below investment grade | secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 600,000
Weighted average duration of MBS portfolio	4 years 2 months 12 days
Collateralized Mortgage Backed Securities
Gain (Loss) on Securities [Line Items]
OTTI on evaluation of securities	$ 0		$ 0