ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES - COVID-19 Impact (Narrative) (Details) - USD ($)		3 Months Ended			9 Months Ended
	Mar. 20, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
COVID-19 Impact [Line Items]
Proceeds From Sale Of Mortgage Backed Securities MBS Categorized As Trading					$ 2,692,230,000	$ 1,948,079,000
Marketable Securities, Realized Gain (Loss)		$ 498,000		$ (5,491,000)	(24,522,000)	(5,135,000)
Gain Loss On Derivative Instruments Held For Trading Purposes Net		4,079,000		(8,648,000)	(87,630,000)	(61,968,000)
Mortgage-backed securities		3,540,367,000			3,540,367,000			$ 3,590,921,000
Outstanding repurchase obligations		3,281,303,000			3,281,303,000			3,448,106,000
Stockholders Equity		376,741,000	$ 308,144,000	391,988,000	376,741,000	391,988,000	$ 345,968,000	395,507,000	$ 359,658,000	$ 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				2,700,000,000
Marketable Securities, Realized Gain (Loss)	$ (31,400,000)				(24,500,000)
Mortgage-backed securities		3,500,000,000	2,900,000,000		3,500,000,000		3,300,000,000	3,600,000,000
Outstanding repurchase obligations		3,300,000,000	2,800,000,000		3,300,000,000		3,200,000,000	3,400,000,000
Stockholders Equity		376,700,000	308,100,000		376,700,000		$ 346,000,000.0	$ 395,500,000
Swap [Member]
COVID-19 Impact [Line Items]
Gain Loss On Derivative Instruments Held For Trading Purposes Net		$ 489,000		$ (9,918,000)	(67,713,000)	$ (36,322,000)
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			$ (45,000,000.0)		$ (54,500,000)