OFFSETTING ASSETS AND LIABILITIES - Offsetting of Liabilties (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	$ 4,216,737	$ 3,628,813
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	4,216,737	3,628,813
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(4,079,105)	(3,536,757)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(102,575)	(78,874)
Net Amount Of Liabilities	35,057	13,182
Repurchase Agreement [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	4,181,680	3,595,586
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	4,181,680	3,595,586
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	(4,079,105)	(3,536,757)
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	(102,575)	(58,829)
Net Amount Of Liabilities	0	0
Swap [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties		24,711
Gross Amount Of Liabilties Offset In The Balance Sheet		0
Net Amount Of Liabilities Presented In The Balance Sheet		24,711
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet		0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet		(19,761)
Net Amount Of Liabilities		4,950
TBA securities [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties		786
Gross Amount Of Liabilties Offset In The Balance Sheet		0
Net Amount Of Liabilities Presented In The Balance Sheet		786
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet		0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet		(284)
Net Amount Of Liabilities		502
Payer Swaption [Member]
Offsetting Liabilities [Line Items]
Gross Amount Of Recognized Liabilties	35,057	7,730
Gross Amount Of Liabilties Offset In The Balance Sheet	0	0
Net Amount Of Liabilities Presented In The Balance Sheet	35,057	7,730
Gross Amount Of Financial Instruments Posted Not Offset in Balance Sheet	0	0
Gross Amounts Of Cash Posted Not Offset In Balance Sheet	0	0
Net Amount Of Liabilities	$ 35,057	$ 7,730