VARIABLE INTEREST ENTITIES - Additional Information (Details)		1 Months Ended			3 Months Ended			6 Months Ended		12 Months Ended
		Feb. 29, 2016 USD ($) loan	Apr. 30, 2015 USD ($) loan	Feb. 28, 2015 USD ($) loan	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)		Jul. 31, 2018 USD ($)	Aug. 31, 2017	Jul. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)
Variable Interest Entity [Line Items]
Exposure to obligations of VIEs					$ 188,000,000			$ 188,000,000
Contractual principal amount of residential mortgage trust debt								86,545,000		$ 349,220,000
Residential mortgage loans	[1]				1,666,157,000			1,666,157,000		1,438,322,000	[2]
Securitized debt of consolidated VIEs	[3]				2,728,692,000			2,728,692,000		2,971,771,000	[2]
Less: General and administration expenses					63,781,000		$ 54,023,000	126,291,000	$ 107,851,000
Other secured financings					3,760,487,000			3,760,487,000		3,837,528,000	[2]
Borrower
Variable Interest Entity [Line Items]
Other secured financings					70,700,000			70,700,000
Borrower | Corporate debt
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility					433,500,000			433,500,000
Consolidated VIEs
Variable Interest Entity [Line Items]
Exposure to obligations of VIEs					1,600,000,000			1,600,000,000
Gain (loss) recognized upon initial consolidation								0
Debt issue costs expensed								800,000
Residential mortgage loans					523,000,000			523,000,000		478,800,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Residential mortgage loans						$ 327,500,000
Securitized debt of consolidated VIEs					257,500,000	279,200,000		257,500,000
Less: General and administration expenses						1,500,000
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Non-Agency mortgage-backed securities						$ 60,900,000
Freddie Mac | Real Estate Investment Trust Two | Consolidated VIEs
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets					484,900,000			484,900,000
Variable interest entity, consolidated, carrying amount, liabilities					449,700,000			449,700,000
Freddie Mac | Real Estate Investment Trust Three | Consolidated VIEs
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets					1,500,000,000			1,500,000,000
Variable interest entity, consolidated, carrying amount, liabilities					1,400,000,000			1,400,000,000
FREMF 2015-KLSF | Freddie Mac | Consolidated VIEs
Variable Interest Entity [Line Items]
Payments to purchase mortgage loans				$ 102,100,000
FREMF 2015-KLSF | Freddie Mac | Real Estate Investment Trust One | Consolidated VIEs
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, liabilities					507,000,000			507,000,000
FREMF 2015-KLSF | Freddie Mac | Floating Rate Multifamily Mortgage Loans
Variable Interest Entity [Line Items]
Number of loans in the underlying portfolio (loan) | loan				11
Face value				$ 1,400,000,000
FREMF 2015-KF07 | Freddie Mac | Consolidated VIEs
Variable Interest Entity [Line Items]
Payments to purchase mortgage loans			$ 89,400,000
FREMF 2015-KF07 | Freddie Mac | Floating Rate Multifamily Mortgage Loans
Variable Interest Entity [Line Items]
Number of loans in the underlying portfolio (loan) | loan			40
Face value			$ 1,200,000,000
FREMF 2016-KLH1 | Freddie Mac | Consolidated VIEs
Variable Interest Entity [Line Items]
Payments to purchase mortgage loans		$ 107,600,000
Face value		112,000,000
Discounted value		$ 4,400,000
FREMF 2016-KLH1 | Freddie Mac | Real Estate Investment Trust One | Consolidated VIEs
Variable Interest Entity [Line Items]
Variable interest entity, consolidated, carrying amount, assets					547,000,000			547,000,000
FREMF 2016-KLH1 | Freddie Mac | Floating Rate Multifamily Mortgage Loans
Variable Interest Entity [Line Items]
Number of loans in the underlying portfolio (loan) | loan		28
Face value		$ 1,500,000,000
FREMF Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance					2,500,000,000			2,500,000,000
Gain (loss) attributable to instrument- specific credit risk								0
Residential mortgage loans					0			0		0
Less: General and administration expenses					0		1,000	0	1,000
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]
Mortgage loans, unpaid principal balance					85,200,000			85,200,000
Contractual principal amount of residential mortgage trust debt								32,900,000
Residential mortgage loans					214,694,000			214,694,000		478,811,000
Less: General and administration expenses					15,000		$ 17,000	29,000	$ 37,000
Other secured financings					0			0		$ 0
Borrower | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Intercompany receivable					70,700,000			70,700,000
Borrower | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity															$ 300,000,000
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]
Transferred loans pledged as collateral for credit facility					208,500,000			208,500,000
Other secured financings					$ 75,100,000			75,100,000
Pingora
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage													100.00%
CMBS Securitization Trust | Consolidated VIEs | Collateralized Mortgage Backed Securities
Variable Interest Entity [Line Items]
Commitments to purchase subordinated tranche in securitization trust								$ 94,000,000
CMBS Securitization Trust | Consolidated VIEs | Collateralized Mortgage Backed Securities | Forecast
Variable Interest Entity [Line Items]
Principal balance of mortgage loans												$ 933,000,000

[1] Includes securitized residential mortgage loans transferred or pledged to a consolidated VIE carried at fair value of $523.0 million and $478.8 million at June 30, 2018 and December 31, 2017, respectively.

[2] Derived from the audited consolidated financial statements at December 31, 2017.

[3] Includes securitized debt of consolidated VIEs carried at fair value of $2.7 billion and $3.0 billion at June 30, 2018 and December 31, 2017, respectively.