SECURITIES - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Mar. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) security segment sale	Jun. 30, 2017 USD ($) sale	Dec. 31, 2017 USD ($) security
Financial Instruments Owned and Pledged as Collateral [Line Items]
Available-for-sale securities, at fair value transferred to held to maturity		$ 50,000,000
Loss on transfer of securities	$ 2,100,000	$ 2,200,000
Number of sales of available for sale securities | sale			29	31
Proceeds from sale of available for sale securities			$ 24,400,000	$ 279,200,000.0
Net gain on sale of available for sale securities			66,000	294,000
Gross unrealized gain on AVS securities			172,000	777,000
Gross unrealized loss on AVS securities			$ 106,000	$ 483,000
Number of portfolio segments | segment			2
Impairment loss realized in consolidated statements of income	$ 0		$ 0
Average loss on securities in an unrealized loss position	0.0297		0.0297		0.0169
Number of securities in an unrealized loss position | security	19		19		18
Repurchase Agreements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost of securities pledged	$ 5,400,000		$ 5,400,000		$ 6,300,000
Securities pledged and available to be sold under repurchase agreements, fair value	5,200,000		5,200,000		6,300,000
Securities pledged to various Federal Reserve Districts relating to deposits of bankruptcy trustees
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost of securities	74,400,000		74,400,000		55,700,000
Fair value of pledged securities	73,400,000		73,400,000		55,500,000
Derivative
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost of securities pledged	3,300,000		3,300,000		3,400,000
Securities pledged and available to be sold under repurchase agreements, fair value	$ 3,200,000		$ 3,200,000		$ 3,300,000