FINANCIAL INSTRUMENTS - Summary of Characteristics of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	$ 130,295,081	$ 105,787,527
Liabilities	114,498,737	91,669,726
Total securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	114,833,580	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	112,124,958	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	768,409	912,673
Credit risk transfer securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	531,322	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	1,135,868	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	64,655	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	208,368	18,516
Total loans, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	4,462,350	4,585,975
Residential mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	1,647,787	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	669,713	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, net
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,144,850	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	7,002,460	3,833,200
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	1,122,588	0
Residential mortgage loans, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,598,374	1,094,831
Commercial mortgage loans, recognized through earnings
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	2,345,120	2,738,369
Commercial mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Assets	936,378	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	101,740,728	81,115,874
Other secured financing
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	4,455,700	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	5,622,801	3,347,062
Mortgages payable
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Liabilities	$ 485,005	$ 511,056