INVESTMENT SECURITIES - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) secuirty	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) secuirty	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0		$ 0
Securities pledged as collateral	972,800,000		972,800,000		$ 688,200,000
Sale of investment securities available for sale			7,012,000	$ 415,486,000
Debt securities, available-for-sale, realized gain (less than)			100,000	1,900,000
Losses from sale of available-for-sale securities (less than during the current quarter end)			0	100,000
Proceeds from sale of equity securities			6,200,000	0
Realized gain on sale of equity investment	3,757,000	$ 0	3,757,000	$ 0
Realized losses on equity securities			0
Unamortized premiums			12,800,000		14,100,000
Unaccreted discounts			1,800,000		1,300,000
Owned investment securities	1,100,000,000		1,100,000,000
Total unrealized losses on securities	$ 41,900,000		$ 41,900,000
Number of securities, below investment grade | secuirty	1		1
Available-for-sale securities, below investment grade, fair value	$ 600,000		$ 600,000
Weighted average duration of MBS portfolio			5 years 7 months 5 days
Collateralized Mortgage Backed Securities
Gain (Loss) on Securities [Line Items]
OTTI on evaluation of securities			$ 0		$ 0