FAIR VALUE - Summary of Assets and Liabilities measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets measured at fair value on recurring basis
Available for sale securities	$ 661,862	$ 759,916
U.S. GSE securities
Assets measured at fair value on recurring basis
Available for sale securities	28,544	56,814
State and municipal obligations
Assets measured at fair value on recurring basis
Available for sale securities	35,172	87,022
U.S. GSE residential mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	92,730	186,901
U.S. GSE residential collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	345,309	307,390
U.S. GSE commercial mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	3,480	5,979
U.S. GSE commercial collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	90,669	48,716
Other asset backed securities
Assets measured at fair value on recurring basis
Available for sale securities	23,522	23,401
Corporate bonds
Assets measured at fair value on recurring basis
Available for sale securities	42,436	43,693
Carrying Amount
Assets measured at fair value on recurring basis
Available for sale securities	661,862	759,916
Financial Assets: Derivatives	9,865	4,546
Financial Liabilities:
Financial liabilities: Derivatives	3,364	1,823
Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets measured at fair value on recurring basis
Available for sale securities	0	0
Financial Assets: Derivatives	0	0
Financial Liabilities:
Financial liabilities: Derivatives	0	0
Significant Other Observable Inputs (Level 2)
Assets measured at fair value on recurring basis
Available for sale securities	661,862	759,916
Financial Assets: Derivatives	9,865	4,546
Financial Liabilities:
Financial liabilities: Derivatives	3,364	1,823
Significant Unobservable Inputs (Level 3)
Assets measured at fair value on recurring basis
Available for sale securities	0	0
Financial Assets: Derivatives	0	0
Financial Liabilities:
Financial liabilities: Derivatives	0	0
Recurring basis | Carrying Amount
Assets measured at fair value on recurring basis
Available for sale securities	661,862	759,916
Recurring basis | Carrying Amount | U.S. GSE securities
Assets measured at fair value on recurring basis
Available for sale securities	28,544	56,814
Recurring basis | Carrying Amount | State and municipal obligations
Assets measured at fair value on recurring basis
Available for sale securities	35,172	87,022
Recurring basis | Carrying Amount | U.S. GSE residential mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	92,730	186,901
Recurring basis | Carrying Amount | U.S. GSE residential collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	345,309	307,390
Recurring basis | Carrying Amount | U.S. GSE commercial mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	3,480	5,979
Recurring basis | Carrying Amount | U.S. GSE commercial collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	90,669	48,716
Recurring basis | Carrying Amount | Other asset backed securities
Assets measured at fair value on recurring basis
Available for sale securities	23,522	23,401
Recurring basis | Carrying Amount | Corporate bonds
Assets measured at fair value on recurring basis
Available for sale securities	42,436	43,693
Recurring basis | Carrying Amount | Derivatives
Assets measured at fair value on recurring basis
Financial Assets: Derivatives	9,865	4,546
Financial Liabilities:
Financial liabilities: Derivatives	3,364	1,823
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets measured at fair value on recurring basis
Available for sale securities	661,862	759,916
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. GSE securities
Assets measured at fair value on recurring basis
Available for sale securities	28,544	56,814
Recurring basis | Significant Other Observable Inputs (Level 2) | State and municipal obligations
Assets measured at fair value on recurring basis
Available for sale securities	35,172	87,022
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. GSE residential mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	92,730	186,901
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. GSE residential collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	345,309	307,390
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. GSE commercial mortgage-backed securities
Assets measured at fair value on recurring basis
Available for sale securities	3,480	5,979
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. GSE commercial collateralized mortgage obligations
Assets measured at fair value on recurring basis
Available for sale securities	90,669	48,716
Recurring basis | Significant Other Observable Inputs (Level 2) | Other asset backed securities
Assets measured at fair value on recurring basis
Available for sale securities	23,522	23,401
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate bonds
Assets measured at fair value on recurring basis
Available for sale securities	42,436	43,693
Recurring basis | Significant Other Observable Inputs (Level 2) | Derivatives
Assets measured at fair value on recurring basis
Financial Assets: Derivatives	9,865	4,546
Financial Liabilities:
Financial liabilities: Derivatives	$ 3,364	$ 1,823