DERIVATIVE INSTRUMENTS - Summary of Fair Value Information about Derivative Assets and Liabilities (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Interest rate swaps, at fair value	$ 26,020,000	$ 48,114,000
Derivative assets	148,178,000	200,503,000	[1]
Interest rate swaps, at fair value	509,485,000	420,365,000
Derivative liabilities	775,980,000	889,750,000	[1]
Futures contracts
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	357,000	0
Derivative assets	357,000
Other derivative liabilities, at fair value	260,354,000	462,309,000
Derivative liabilities	260,354,000	462,309,000
Purchase commitments
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	2,434,000	844,000
Derivative assets	2,434,000	844,000
Other derivative liabilities, at fair value	478,000	33,000
Derivative liabilities	478,000	33,000
Interest rate swaptions
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	8,250,000	7,216,000
Derivative assets	8,250,000	7,216,000
TBA derivatives
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	106,960,000	141,688,000
Derivative assets	106,960,000	141,688,000
Other derivative liabilities, at fair value	5,212,000	0
Derivative liabilities	5,212,000
Credit derivatives
Derivatives, Fair Value [Line Items]
Other derivative assets, at fair value	4,157,000	2,641,000
Derivative assets	4,157,000	2,641,000
Other derivative liabilities, at fair value	451,000	7,043,000
Derivative liabilities	451,000	7,043,000
Notional amount of derivative assets	45,000,000.0	30,000,000.0
Notional	$ 346,000,000	$ 451,000,000

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