OFFSETTING ASSETS AND LIABILITIES - Offsetting of Assets (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 37,422,000	$ 17,160,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	37,422,000	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	(9,106,000)	(3,415,000)
Net Amount Of Assets	28,316,000	13,745,000
Swap [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	28,030,000	13,745,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	28,030,000	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	28,030,000	13,745,000
TBA securities [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,035,000	10,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	2,035,000	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	(1,749,000)	(10,000)
Net Amount Of Assets	286,000	0
Payer Swaption [Member]
Offsetting Assets [Line Items]
Derivative Asset, Fair Value, Gross Asset	7,357,000	3,405,000
Gross Amount Of Assets Offset In The Balance Sheet	0	0
Net Amount Of Assets Presented In The Balance Sheet	7,357,000	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	(7,357,000)	(3,405,000)
Net Amount Of Assets	$ 0	$ 0