OFFSETTING ASSETS AND LIABILITIES - Offsetting of Assets (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 36,179,000	$ 17,160,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	36,179,000	17,160,000
Gross Amounts Of Financial Instruments Received Not Offset In Balance Sheet	0	0
Gross Amounts Of Cash Collateral Received Not Offset In Balance Sheet	(7,169,000)	(3,415,000)
Net Amount Of Assets	29,010,000	13,745,000
Swap [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	25,107,000	13,745,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	25,107,000	13,745,000
Gross Amounts Of Financial Instruments Received Not Offset In Balance Sheet	0	0
Gross Amounts Of Cash Collateral Received Not Offset In Balance Sheet	0	0
Net Amount Of Assets	25,107,000	13,745,000
TBA securities [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	16,000	10,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	16,000	10,000
Gross Amounts Of Financial Instruments Received Not Offset In Balance Sheet	0	0
Gross Amounts Of Cash Collateral Received Not Offset In Balance Sheet	0	(10,000)
Net Amount Of Assets	16,000	0
Payer Swaption [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	11,056,000	3,405,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	11,056,000	3,405,000
Gross Amounts Of Financial Instruments Received Not Offset In Balance Sheet	0	0
Gross Amounts Of Cash Collateral Received Not Offset In Balance Sheet	(7,169,000)	(3,405,000)
Net Amount Of Assets	$ 3,887,000	$ 0