FINANCIAL INSTRUMENTS - Summary of Characteristics of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	$ 128,956,120	$ 105,787,527	[1]
Liabilities	113,732,416	91,669,726	[1]
Total securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	116,094,061	92,623,788
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	113,670,254	89,840,322
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	792,270	912,673
Credit risk transfer securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	474,765	552,097
Non-agency mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	1,015,921	1,161,938
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	65,215	138,242
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	75,636	18,516
Total loans, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	3,946,614	4,585,975
Residential mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	1,219,402	1,359,806
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	611,429	1,296,803
Commercial loans held for sale, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	42,184
Corporate debt, held for investment
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,115,783	1,887,182
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	4,688,144	3,833,200
Residential mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,376,731	1,094,831
Commercial mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	1,501,408	2,738,369
Commercial mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	810,005	0
Reverse repurchase agreements
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	0	650,040
Repurchase agreements
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	102,682,104	81,115,874
Other secured financing
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	4,466,030	4,183,311
Debt issued by securitization vehicles
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	3,856,082	3,347,062
Mortgages payable
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	$ 485,657	$ 511,056

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