DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Interest rate swaps, at fair value	$ 5,440,000	$ 48,114,000
Derivative assets	75,142,000	200,503,000	[1]
Interest rate swaps, at fair value	677,295,000	420,365,000
Derivative liabilities	1,043,197,000	889,750,000	[1]
Futures contracts
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	20,388,000	0
Derivative assets	20,388,000
Other derivative liabilities, at fair value	363,165,000	462,309,000
Derivative liabilities	363,165,000	462,309,000
Purchase commitments
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	3,957,000	844,000
Derivative assets	3,957,000	844,000
Other derivative liabilities, at fair value	896,000	33,000
Derivative liabilities	896,000	33,000
Interest rate swaptions
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	5,444,000	7,216,000
Derivative assets	5,444,000	7,216,000
TBA derivatives
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	34,889,000	141,688,000
Derivative assets	34,889,000	141,688,000
Other derivative liabilities, at fair value	1,432,000	0
Derivative liabilities	1,432,000
Credit derivatives
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	5,024,000	2,641,000
Derivative assets	5,024,000	2,641,000
Other derivative liabilities, at fair value	409,000	7,043,000
Derivative liabilities	409,000	7,043,000
Notional amount of derivative assets	30,000,000.0	30,000,000
Notional	$ 336,000,000.0	$ 451,000,000.0

[1] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.