ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES - COVID-19 Impact (Narrative) (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
COVID-19 Impact [Line Items]
Proceeds From Sale Of Mortgage Backed Securities MBS Categorized As Trading				$ 2,023,334,000	$ 1,689,747,000
Marketable Securities, Realized Gain (Loss)	$ 3,360,000		$ 112,000	(25,020,000)	355,000
Gain Loss On Derivative Instruments Held For Trading Purposes Net	(8,851,000)		(34,288,000)	(91,709,000)	(53,320,000)
Mortgage-backed securities	3,304,761,000			3,304,761,000		$ 3,590,921,000
Outstanding repurchase obligations	3,174,739,000			3,174,739,000		3,448,106,000
Stockholders Equity	345,968,000	$ 308,144,000	$ 359,658,000	345,968,000	$ 359,658,000	395,507,000	$ 340,414,000	$ 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		2,000,000,000
Marketable Securities, Realized Gain (Loss)		(31,400,000)		(25,000,000)
Mortgage-backed securities	3,300,000,000	2,900,000,000		3,300,000,000		3,600,000,000
Outstanding repurchase obligations	3,200,000,000	2,800,000,000		3,200,000,000		3,400,000,000
Stockholders Equity	$ 346,000,000	308,100,000		346,000,000		$ 395,500,000
Swap [Member] | COVID-19 [Member]
COVID-19 Impact [Line Items]
Terminated Swap Positions Notional				1,200,000,000
Gain Loss On Derivative Instruments Held For Trading Purposes Net		$ (45,000,000)		$ (54,500,000)