FINANCIAL INSTRUMENTS - Summary of Characteristics of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	$ 85,369,589	$ 88,455,103	[1]
Liabilities	71,301,994	74,433,307	[1]
Total securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	71,849,437	75,652,396
Agency mortgage-backed securities, recognized through comprehensive income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	69,148,257	73,562,972
Agency mortgage-backed securities, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	488,972	504,087
Residential credit risk transfer securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	930,983	532,403
Non-agency mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	1,277,104	972,192
Commercial real estate debt investment, securities, recognized through other comprehensive income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	31,603
Commercial real estate debt investment, securities, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	45,254
Commercial real estate debt investment, credit risk transfer securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	4,121	3,885
Total loans, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,603,343	3,083,821
Residential mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	528,868	345,810
Commercial real estate debt and preferred equity, held for investment
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	498,081
Corporate debt held for investment, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,074,475	2,239,930
Total assets transferred or pledged to securitization vehicles
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	3,768,922	6,910,020
Mortgage-backed securities, recognized through other comprehensive income
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	598,118	620,347
Residential mortgage loans, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	3,170,804	3,249,251
Commercial mortgage loans, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	2,166,073
Commercial mortgage loans, amortized cost
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	874,349
Repurchase agreements
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	61,202,477	64,825,239
Other secured financing
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	922,605	917,876
Debt issued by securitization vehicles
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	3,044,725	5,652,982
Participations issued
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	180,527	39,198
Mortgages payable
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	$ 0	$ 426,256

[1] Derived from the audited consolidated financial statements at December 31, 2020.