VARIABLE INTEREST ENTITIES - Additional Information (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jul. 31, 2018	Dec. 31, 2017	Aug. 31, 2017	Jul. 31, 2017
Variable Interest Entity [Line Items]
Contractual principal amount of residential mortgage trust debt				$ 516,180,000
Residential mortgage loans	$ 4,181,878,000			4,181,878,000			$ 2,999,148,000
Securitized debt of consolidated VIEs	3,799,542,000			3,799,542,000			2,971,771,000
Mortgage-backed securities	91,338,611,000			91,338,611,000			92,563,572,000
Costs incurred in connection with securitization	126,509,000		$ 57,016,000	252,800,000	$ 164,867,000
Other secured financings	4,108,547,000			4,108,547,000			3,837,528,000
Variable interest entity, ownership percentage								100.00%
Borrower
Variable Interest Entity [Line Items]
Other secured financings	300,000,000			300,000,000
Borrower | Corporate debt
Variable Interest Entity [Line Items]
Loans, pledged assets	443,600,000			443,600,000
Consolidated VIEs
Variable Interest Entity [Line Items]
Exposure to obligations of VIEs	1,600,000,000			1,600,000,000
Consolidated VIEs | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Mortgage-backed securities	275,800,000			275,800,000			0
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Residential mortgage loans		$ 327,500,000				$ 383,400,000
Securitized debt of consolidated VIEs	530,100,000	588,100,000		530,100,000
Costs incurred in connection with securitization	1,800,000	1,500,000
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Mortgage-backed securities		$ 122,500,000
FREMF Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance	3,500,000,000			3,500,000,000
Gain (loss) attributable to instrument- specific credit risk				0
Residential mortgage loans	0			0			0
Costs incurred in connection with securitization	0		(1,000)	0	0
Other secured financings	0			0			0
FREMF Trusts | Consolidated VIEs | 90 days or more past due
Variable Interest Entity [Line Items]
Loans 90 days or more past due or on nonaccrual status	0			0
Residential Mortgage Loan Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Contractual principal amount of residential mortgage trust debt				30,400,000
Residential mortgage loans	107,764,000			107,764,000			478,811,000
Costs incurred in connection with securitization	21,000		$ 34,000	50,000	$ 71,000
Other secured financings	0			0			$ 0
Borrower | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Principal receivable	300,000,000			300,000,000
Borrower | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity	400,000,000			400,000,000
Borrower | Consolidated VIEs | July 2017 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity									$ 150,000,000
Borrower | Consolidated VIEs | Corporate debt | July 2017 Credit Facility
Variable Interest Entity [Line Items]
Loans, pledged assets	235,600,000			235,600,000
Other secured financings	$ 150,000,000			150,000,000
CMBS Securitization Trust | Consolidated VIEs | Total securities
Variable Interest Entity [Line Items]
Commitments to purchase subordinated tranche in securitization trust				$ 94,000,000