ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES - COVID-19 Impact (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
COVID-19 Impact [Line Items]
Proceeds From Sale Of Mortgage Backed Securities MBS Categorized As Trading	$ 1,808,867,000	$ 655,359,000
Marketable Securities, Realized Gain (Loss)	(28,380,000)	243,000
Gain Loss On Derivative Instruments Held For Trading Purposes Net	(82,858,000)	(19,032,000)
Mortgage-backed securities	2,948,797,000		$ 3,590,921,000
Outstanding repurchase obligations	2,810,250,000		3,448,106,000
Stockholders Equity	308,144,000	340,414,000	$ 395,507,000	$ 308,144,000	$ 336,079,000
COVID-19 [Member]
COVID-19 Impact [Line Items]
Proceeds From Sale Of Mortgage Backed Securities MBS Categorized As Trading	1,100,000,000
Marketable Securities, Realized Gain (Loss)	(31,400,000)
Swap [Member]
COVID-19 Impact [Line Items]
Gain Loss On Derivative Instruments Held For Trading Purposes Net	(60,623,000)	$ (2,295,000)
Swap [Member] | COVID-19 [Member]
COVID-19 Impact [Line Items]
Terminated Swap Positions Notional	860,000,000
Gain Loss On Derivative Instruments Held For Trading Purposes Net	$ (45,000,000)