FINANCIAL INSTRUMENTS - Summary of Characteristics of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	$ 98,832,997	$ 101,760,050	[1]
Liabilities	85,059,141	86,888,477	[1]
Total agency mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	86,593,058	90,551,763
Residential mortgage backed securities, recognized through comprehensive income
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	85,593,158	89,426,437
Residential mortgage backed securities, recognized trough earnings
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	999,900	1,125,326
Credit risk transfer securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	563,796	651,764
Non-agency mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	1,006,785	1,097,294
Residential mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	1,666,157	1,438,322
Total commercial real estate debt investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	2,857,463	3,089,108
Commercial real estate debt investment, loans, recognized though earnings
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	2,542,413	2,826,357
Commercial real estate debt investment, securities, recognized through comprehensive income
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	204,319	244,636
Commercial real estate debt investment, securities, recognized through earnings
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	110,731	18,115
Commercial real estate debt and preferred equity, held for investment
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	1,251,138	1,029,327
Loans held for sale, net
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	42,458	0
Corporate debt
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	1,256,276	1,011,275
Reverse repurchase agreements
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Assets	259,762	0
Repurchase agreements
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Liabilities	75,760,655	77,696,343
Other secured financing
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Liabilities	3,760,487	3,837,528
Securitized debt of consolidated VIEs
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Liabilities	2,728,692	2,971,771
Mortgages payable
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Liabilities	$ 309,878	$ 309,686

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