DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Interest rate swaps	$ 1,199,000	$ 48,114,000
Total derivative assets	113,556,000	200,503,000
Interest rate swaps	706,862,000	420,365,000
Total derivative liabilities	803,866,000	889,750,000
Futures Contracts
Derivatives, Fair Value [Line Items]
Derivative assets	77,889,000	0
Total derivative assets	77,889,000
Derivative liabilities	84,781,000	462,309,000
Total derivative liabilities	84,781,000	462,309,000
Purchase Commitments
Derivatives, Fair Value [Line Items]
Derivative assets	2,050,000	844,000
Total derivative assets	2,050,000	844,000
Derivative liabilities	907,000	33,000
Total derivative liabilities	907,000	33,000
Interest Rate Swaptions
Derivatives, Fair Value [Line Items]
Derivative assets	11,580,000	7,216,000
Total derivative assets	11,580,000	7,216,000
TBA Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	15,181,000	141,688,000
Total derivative assets	15,181,000	141,688,000
Derivative liabilities	11,316,000	0
Total derivative liabilities	11,316,000
Notional amount of derivative assets	6,899,000,000
Credit Derivatives
Derivatives, Fair Value [Line Items]
Derivative assets	5,657,000	2,641,000
Total derivative assets	5,657,000	2,641,000
Derivative liabilities	0	7,043,000
Total derivative liabilities		7,043,000
Notional amount of derivative assets	10,000,000.0	30,000,000.0
Notional amount	$ 345,000,000.0	$ 451,000,000.0